Cerium-containing nanoparticles

ABSTRACT

A process for making cerium-containing oxide nanoparticles includes providing an aqueous reaction mixture containing a source of cerous ion, optionally a source of one or more metal ions (M) other than cerium, a source of hydroxide ion, at least one monoether carboxylic acid nanoparticle stabilizer wherein the molar ratio of said monoether carboxylic acid nanoparticle stabilizers to total metal ions is greater than 0.2, and an oxidant at an initial temperature in the range of about 20° C. to about 95° C. Temperature conditions are provided effective to enable oxidation of cerous ion to ceric ion, thereby forming a product dispersion of cerium-containing oxide nanoparticles, optionally containing one or more metal ions (M), Ce 1-x M x O 2-δ , wherein “x” has a value from about 0.0 to about 0.95. The nanoparticles may have a mean hydrodynamic diameter from about 1 nm to about 50 nm, and a geometric diameter of less than about 45 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT/US07/077,545, METHODOF PREPARING CERIUM DIOXIDE NANOPARTICLES, filed Sep. 4, 2007; in thenames of Kenneth J. Reed et al., which claims the benefit of priorityfrom: Provisional Application Ser. No. 60/824,514, CERIUM-CONTAININGFUEL ADDITIVE, filed Sep. 5, 2006; Provisional Application Ser. No.60/911,159, REVERSE MICELLAR FUEL ADDITIVE COMPOSITION, filed Apr. 11,2007; and Provisional Application Ser. No. 60/938,314, REVERSE MICELLARFUEL ADDITIVE COMPOSITION, filed May 16, 2007. This application is alsoa continuation-in-part of PCT/US2008/087133, FUEL ADDITIVE CONTAININGLATTICE ENGINEERED CERIUM DIOXIDE NANOPARTICLES, filed Dec. 17, 2008; inthe names of Kenneth J. Reed et al. This application is also related to:U.S. patent application Ser. No. 12/549,776, PROCESS FOR SOLVENTSHIFTING A NANOPARTICLE DISPERSION, filed Aug. 28, 2009. The disclosureof all of these applications is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates in general to cerium-containingnanoparticles and, in particular embodiments, to cerium oxidenanoparticles that further contain one or more metals (M) other thancerium, and to a method for preparing such particles. Thesenanoparticles are useful, in part, as components of fuel additivecompositions.

BACKGROUND OF THE INVENTION

Cerium-containing oxide nanoparticles have many current industrial uses,as well as many potential applications in the future. They are wellknown as important components in solid oxide fuel cells, three-wayautomotive exhaust catalysts, automotive fuel borne catalysts, andultra-violet sun blockers, to name just a few. Its utility is oftenattributed to its solid state redox chemistry, resulting from therelatively facile Ce³⁺/Ce⁴⁺ electrochemical conversion. This allowsnanoceria, for example, to store oxygen under oxidizing conditions,wherein Ce³⁺ is converted to Ce⁴⁺, and to release oxygen under reducingconditions, wherein Ce⁴⁺ is converted to Ce³⁺ and oxygen vacancies arecreated, a property commonly referred to as its oxygen storage capacity(OSC). As an automotive fuel borne catalyst, the ability of nanoceria tostore and release oxygen in a diesel engine combustion chamber, wherebylocal inhomogeneities in the fuel/oxygen mixture are reduced, isbelieved to produce a more complete burn, thereby generating more powerwith reduced soot and toxic gas emissions.

Many of these end use applications benefit directly from the smallparticle size of nanoceria due to increased surface area and enhancedreactivity. There are many synthetic methods for the production of metaloxides, including aqueous or organic precipitation, hydrothermalprecipitation, spray precipitation, chemical vapor deposition, andplasma deposition techniques. Aqueous precipitation methods areparticularly favored in cases where high through-put is desired, whereina relatively large amount of product is to be produced. However,conventional metal oxide precipitation processes typically include themultiple steps of reactant delivery, particle precipitation, isolation,washing, drying, impregnation, calcination (heating to 400-1000° C. forseveral hours), grinding, milling and particle size classification,among others. Alternatively, direct methods seek to produce a dispersion(suspension) of the final particles directly, thereby avoiding the time,cost and potential contamination inherent in the isolation, drying,calcination, grinding, milling and classification steps. For many enduse applications, however, these direct methods present the additionalchallenge of maintaining dispersion stability (preventing aggregation orclumping of particles) during subsequent washing, handling and storageof the dispersed product particles.

Aqueous precipitation methods for the direct preparation of nanoceriaare described in U.S. Pat. No. 5,389,352; U.S. Pat. No. 5,938,837 andU.S. Patent Appl. No. 2007/0215378. The basic precipitation processdescribed in these references involves adding a cerium (III) salt and abase, such as ammonium hydroxide, and converting the cerium (III) saltinto a ceria (CeO₂) precipitate. In some cases an oxidant, such ashydrogen peroxide (H₂O₂) was also included.

Wang, U.S. Pat. No. 5,389,352, describes the reaction of cerous nitratewith ammonia at high temperatures (above 100° C.) in a closed containerfor 24 hours. These hydrothermal precipitations produce a slurry ofceria, evidence of the instability of the particle dispersions.Alternatively, a room temperature reaction of H₂O₂, cerous nitrate andammonia over a 4 hours period is described as producing a powder withaverage crystallite size of about 7 nanometers (nm). However, there isno description of the actual agglomerated particle size, as would berevealed by a transmission electron microscopy (TEM) analysis, or ahydrodynamic diameter measurement by a dynamic light scatteringtechnique. There is also no teaching of the use of a stabilizer additiveto improve dispersion stability, nor any suggestion of how to reduce thetime of the reaction.

Hanawa, U.S. Pat. No. 5,938,837, describes the precipitation of ceriafrom an aqueous solution based reaction of cerous nitrate and ammonia ata pH range between 5 and 10, preferably between 7 and 9, along with theuse of a carefully timed temperature ramp up to 70-100° C. within 10minutes of initial mixing of the reactants. It is evident that theseparticle dispersions have very poor stability as a slurry of particlesis produced. While a crystallite size of about 20 nm was determined fromX-ray Diffraction peak widths and confirmed by TEM analysis, theparticles are highly agglomerated as evidence by the TEM image of FIG.2, which was taken after a deagglomeration step. There is no teaching ofthe use of an oxidant, nor any suggestion to employ a stabilizeradditive to reduce the particle agglomeration or to improve thedispersion stability.

Zhou et al., U.S. Pat. Appl. 2003/0215378, describes the aqueousprecipitation of slurries of cerium dioxide resulting from the reactionof cerium nitrate and ammonium hydroxide during which oxygen is bubbledthrough the reaction mixture. The basic process followed is to form aprecipitate, and then to filter and dry the precipitate. While theprimary crystallite sizes are quite small (3-100 nm), the particles aresubstantially aggregated as shown in TEM images taken only after thesamples were prepared by ultrasonically dispersing the powder inethanol. There is no suggestion to employ an oxidant stronger thanmolecular oxygen. There is no suggestion to employ a stabilizer additiveto reduce the particle aggregation or to improve the particle dispersionstability.

Cuif et al., U.S. Pat. No. 6,133,194, describes the use of anionicsurfactants, non-ionic surfactants, polyethylene glycols, carboxylicacids, and carboxylate salts as additives in a conventional aqueousprecipitation or co-precipitation process involving cerium solutions,zirconium solutions, base and optionally an oxidizing agent, at a pHpreferably greater than about 7, wherein after the reaction stage, mixedhydroxides, such as (Ce,Zr)(OH)₄, are precipitated, the solidprecipitate is recovered and separated from the mother liquor byconventional solid/liquid separation techniques such as decantation,drying, filtration and/or centrifugation, then washed, calcined at aminimum temperature of 400° C., a temperature high enough to ensureremoval of carbonaceous remnants from the oxide, hydroxide or carbonate.Many additives are disclosed for addition to the reaction mixture fromwhich the mixed hydroxides are precipitated, isolated, washed andcalcined. Many alkoxylated compounds are disclosed for use in thewashing or impregnation, preferably in the form of a wet cake, followedby calcination. There is no disclosure of monoether carboxylic acids, orsalts thereof, as an additive. Furthermore there is no suggestion to useany of the additives disclosed therein in a direct preparation method ofmaking metal oxide, hydroxide or carbonate particles with a goal ofreducing particle size or maintaining or improving particle dispersionstability.

Poncelet et al., FR 2885308, describe the use of polyether carboxylicacids (2-(2-methoxyethoxy) acetic acid (MEAA) and2-(2-(2-methoxyethoxy)ethoxy) acetic acid (MEEAA)) and the monoethercarboxylic acid (3-methoxypropionic acid (MPA)) as an additive in thepreparation of method of Cuif et al. (U.S. Pat. No. 6,133,194) forcerium oxide, zirconium oxide or a mixed oxide of cerium and zirconium.Example 3 shows that use of a specific monoether carboxylic acid,3-methoxypropionic acid (MPA), in the preparation of cerium oxideAmmonium hydroxide is added to a solution containing a mixture of MPAand cerous nitrate in the molar ratio of 0.16 MPA to cerium ion. Theresulting product was a suspension (dispersion) of cerium oxideparticles. The size of the aggregates formed is reported as having ahydrodynamic diameter of 50-60 nm. Furthermore, the specificationclearly states that the alkoxy carboxylic acid/metallic oxide molarratio is between 0.01 and 0.2. Preferably the alkoxy carboxylicacid/metallic oxide ratio is between 0.05 and 0.15. There is nosuggestion in Poncelet et al. (FR 2885308) to employ an oxidant additivesuch as hydrogen peroxide.

There is a need to provide small nanoparticles of metal oxides, such ascerium oxides and homogeneously doped cerium oxides, and to providerobust, cost-effective methods for their preparation. To date, thesmallest aggregate size achieved using a monoether carboxylic acidstabilizer in an aqueous preparation of cerium oxide is only 50-60 nm.There is a need to provide aqueous dispersions of metal oxidenanoparticles, such as cerium oxides and homogeneously doped ceriumoxides, with excellent dispersion stability, particularly when thepolarity of the solvent is reduced to improve the compatibility of thedispersion with a hydrocarbon diluent/fuel, such as kerosene, dieselfuel or biodiesel fuel. There is a need to provide fuel additives withimproved fuel efficiency, reduced toxic gas and particulate emissions,and reduced engine conditioning time before the benefits of the fueladditive are realized.

SUMMARY OF THE INVENTION

The present invention is directed to a process for makingcerium-containing oxide nanoparticles, optionally containing one or moremetal ions (M) other than cerium, that comprises: (a) providing anaqueous reaction mixture comprising a source of cerous ion and,optionally a source of one or more metal ions (M) other than cerium,wherein said sources of metal ions are introduced concurrently, a sourceof hydroxide ion, at least one monoether carboxylic acid nanoparticlestabilizer, wherein the molar ratio of the monoether carboxylic acidnanoparticle stabilizers to total metal ions is greater than 0.2, and anoxidant at an initial temperature in the range of about 20° C. to about95° C.; and (b) providing temperature conditions effective to enableoxidation of cerous ion to ceric ion, thereby forming a productdispersion comprising nanoparticles of cerium oxide, CeO_(2-δ), or dopedcerium oxide, Ce_(1-x)M_(x)O_(2-δ), wherein “x” has a value from about0.001 to about 0.95, and 8 has a value from about 0.0 to about 0.5. Thecerium oxide nanoparticles thus obtained may have a cubic fluoritestructure, a mean hydrodynamic diameter in the range of about 1 nm toabout 50 nm, and a geometric diameter in the range from about 1 nm about45 nm.

In particular embodiments, the monoether carboxylic acid ismethoxyacetic acid, and the cerium-containing oxide nanoparticlescontain one or more metal ions (M) other than cerium,Ce_(1-x)M_(x)O_(2-δ), wherein “x” has a value from about 0.001 to about0.95, and δ has a value from about 0.0 to about 0.5. In particularembodiments, the metal ions M are zirconium, iron, palladium, orcombinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are, respectively, a TEM image and a particlesize-frequency analysis by TEM of isothermally precipitated CeO_(2-δ)nanoparticles, prepared by a triple jet process as described in Example4.

FIGS. 2A and 2B are, respectively, a TEM image and a particlesize-frequency analysis by TEM of isothermally prepared Fe-containingCeO_(2-δ) nanoparticles, prepared as described in Example 5.

FIGS. 3A and 3B are, respectively, a TEM image and a particlesize-frequency analysis by TEM of isothermally prepared Zr-containingCeO_(2-δ) nanoparticles, prepared as described in Example 6.

FIGS. 4A and 4B are respectively, a TEM image and a particlesize-frequency analysis by TEM of isothermally prepared CeO_(2-δ)nanoparticles containing Zr and Fe, prepared as described in Example 7a.FIG. 4C are X-ray diffraction spectra of isothermally prepared CeO_(2-δ)nanoparticles and of isothermally prepared CeO_(2-δ) nanoparticlescontaining Zr and Fe, prepared as described in Example 7a.

FIG. 5A is a TEM image of a approximately 0.8 micrometer iron containingoxide particle prepared as described in Example 8; FIG. 5B is anelectron diffraction pattern of these micron sized iron containing oxideparticles prepared as described in Example 8.

FIG. 6 includes Table 3 containing particle size, OSC and rate resultsfor homogeneously doped cerium oxide nanoparticle variations.

FIG. 7A is a high resolution TEM image of the nanoparticles ofhomogeneously prepared Ce_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ), prepared asdescribed in Example 17.

FIG. 7B is an electron diffraction pattern ofCe_(0.35)Zr_(0.5)Fe_(0.50)O_(2-δ), prepared as described in Example 17.

FIG. 7C is a powder x-ray diffraction pattern of homogeneously preparedCe_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ), prepared as described in Example 17,superimposed on pure CeO₂ (line spectrum).

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that elements not specifically shown or describedmay take various forms well known to those skilled in the art. Theinvention is defined by the claims.

In this disclosure, the term “metal” in referring to elements of thePeriodic Table includes all elements other than those of the followingatomic numbers: 1-2, 5-10, 14-18, 33-36, 52-54, 85 and 86.

The term “transition metal” is understood to encompass the 40 chemicalelements of atomic number 21 to 30, 39 to 48, 72 to 80, which areincluded in Periods 4, 5, 6, respectively, of the Periodic Table.

The term “rare earth metal” is understood to encompass the 15 chemicalelements of atomic number 57 to 71, which are included in Period 5 ofthe Periodic Table.

The term “alkali metal” is understood to encompass the 6 chemicalelements forming Group 1 of the Periodic Table, those of atomic number3, 11, 19, 37, 55, and 87.

The term “alkaline earth metal” is understood to encompass the 6chemical elements forming Group 2 of the Periodic Table, those of atomicnumber 4, 12, 20, 38, 56, and 88.

Nanoparticles are particles having a mean diameter of less than about100 nm. The size of the resulting cerium-containing oxide particles canbe determined by dynamic light scattering, a measurement technique fordetermining the hydrodynamic diameter of the particles. The hydrodynamicdiameter is typically slightly larger than the geometric diameter of theparticle because it includes both the native particle size and thesolvation shell surrounding the particle. When a beam of light passesthrough a colloidal dispersion, the particles or droplets scatter someof the light in all directions. When the particles are very smallcompared with the wavelength of the light, the intensity of thescattered light is uniform in all directions (Rayleigh scattering). Ifthe light is coherent and monochromatic as, for example, from a laser,it is possible to observe time-dependent fluctuations in the scatteredintensity, using a suitable detector such as a photomultiplier capableof operating in photon counting mode. These fluctuations arise from thefact that the particles are small enough to undergo random thermalBrownian motion, and the distance between them is therefore constantlyvarying. Constructive and destructive interference of light scattered byneighboring particles within the illuminated zone gives rise to theintensity fluctuation at the detector plane, which, because it arisesfrom particle motion, contains information about this motion. Analysisof the time dependence of the intensity fluctuation can therefore yieldthe diffusion coefficient of the particles from which, via the StokesEinstein equation and the known viscosity of the medium, thehydrodynamic radius or diameter of the particles can be calculated.Alternatively, the geometric diameter of a nanoparticle may bedetermined by analysis of TEM images.

Although nominally described as “cerium oxide” or “cerium dioxide”, itis understood by one skilled in the chemical arts, that the actualoxidic anions present may comprise oxide anions or hydroxide anions, ormixtures thereof, such as hydrated oxide phases (e.g. oxyhydroxide). Inaddition, compositions of matter comprised of solid solutions ofmultivalent cations are often termed non-stoichiometric solids. Thus,for oxide phases comprised of metal cations of multiple oxidationstates, it is understood that the total amount of oxidic anions presentwill be determined by the specific amounts of the various oxidationstates of the metal cations present (e.g. Ce³⁺ and Ce⁴⁺), such thatcharge neutrality is maintained. For non-stoichiometric phases nominallydescribed as metal dioxides, this is embodied in the chemical formulaMO_(2-δ), wherein the value of δ (delta) may vary. For cerium oxides,CeO_(2-δ), the value of δ (delta) typically ranges from about 0.0 toabout 0.5, the former denoting cerium (IV) oxide, CeO₂, the latterdenoting cerium (III) oxide, CeO_(1.5) (alternatively denoted Ce₂O₃).

In one particular embodiment, homogeneously doped cerium dioxidenanoparticles of the invention have a median or mean diameter rangingfrom 1.5 to 8 nm. In another embodiment, the median or mean diameterranges from 2 to 4 nm. In still another embodiment, the median or meandiameter ranges from 2 to 3 nm.

The term “doped” particle refers to a particle containing one or moreforeign or dopant ions present in concentrations greater than wouldnormally be present as impurities. Generally, and as used herein, adopant is present in concentrations ranging from about 0.1 percent toabout 95 percent. Above 50% substitutional doping, the roles of host andguest ions become transposed. Doping of cerium dioxide with a metal ionmay be described in general by the formula Ce_(1-x)M_(x)O_(2-δ), whereinx varies from about 0.001 to about 0.95, and δ varies from about 0.0 toabout 0.5 in order to maintain charge neutrality. It is understood thatthe value of δ may be less than zero for metal dopant ions with a formalvalence state greater than 4+. Combinations of dopant metals are alsoconsidered. In particular embodiments, the transition metals are Zr, Fe,and Pd; the rare earth metals are La or Y, or any combination thereof.Doping of cerium dioxide to improve ionic transport, reaction efficiencyand other properties is disclosed in, for example, U.S. Pat. Nos.6,752,979; 6,413,489; 6,869,584; 7,169,196 B2; 7,384,888B2; and U.S.Patent Appl. Publ. No. 2005/0152832. Structured doping of cerium dioxideis described in commonly assigned U.S. Provisional Application Ser. No.61/311,416, STRUCTURED CATALYTIC NANOPARTICLES AND METHOD OFPREPARATION, filed Mar. 8, 2010. Some alternative terms commonly used inplace of “doped” are “substituted”, “mixed metal” and “latticeengineered.”

The term “homogeneously doped cerium oxide” nanoparticle refers to ananoparticle prepared by a process wherein the sources of the dopantmetal ions and cerium ions are introduced concurrently into the reactionmixture. The sources of the various metal ions may, for example, becomixed into the same metal salt solution, or one or more the variousmetals may be dissolved in separate solutions and then addedsimultaneously with the addition of the other metal ions to the reactionmixture, for example, through separate jets. Some alternative termscommonly used in place of “homogeneously doped” are “continuouslydoped”, “uniformly doped,” and “unstructured doped”.

In accordance with one embodiment of the invention, a method ofproducing cerium oxide nanoparticles comprises: (a) providing an aqueousreaction mixture comprising a source of cerous ion, a source ofhydroxide ion, at least one monoether carboxylic acid nanoparticlestabilizer, wherein the molar ratio of the monoether carboxylic acidnanoparticle stabilizers to total cerium ions is greater than 0.2, andan oxidant at an initial temperature in the range of about 20° C. toabout 95° C.; and (b) providing temperature conditions effective toenable oxidation of cerous ion to ceric ion, thereby forming a productdispersion comprising cerium oxide nanoparticles, CeO_(2-δ). Inparticular embodiments the cerium oxide nanoparticles thus obtained havea cubic fluorite structure, a mean hydrodynamic diameter in the range ofabout 1 nm to about 50 nm, and a geometric diameter in the range fromabout 1 nm to about 45 nm.

In accordance with another embodiment of the invention, a method ofproducing doped cerium oxide nanoparticles comprises: (a) providing anaqueous reaction mixture comprising a source of cerous ion and a sourceof one or more metal ions (M) other than cerium, wherein said sources ofmetal ions are introduced concurrently, a source of hydroxide ion, atleast one monoether carboxylic acid nanoparticle stabilizer, wherein themolar ratio of the monoether carboxylic acid nanoparticle stabilizers tototal metal ions is greater than 0.2, and an oxidant at an initialtemperature in the range of about 20° C. to about 95° C.; and (b)providing temperature conditions effective to enable oxidation of cerousion to ceric ion, thereby forming a product dispersion comprisinghomogeneously doped cerium oxide nanoparticles, Ce_(1-x)M_(x)O_(2-δ),wherein “x” has a value from about 0.001 to about 0.95, and δ has avalue from about 0.0 to about 0.5. In particular embodiments the dopedcerium oxide nanoparticles thus obtained have a cubic fluoritestructure, a mean hydrodynamic diameter in the range of about 1 nm toabout 50 nm, and a geometric diameter in the range from about 1 nm toabout 45 nm.

Sources of hydroxide ion include alkali metal hydroxides, such as sodiumor potassium hydroxide, and ammonium hydroxide. Alternative sources ofhydroxide ion include basic solutions of carbonate, bicarbonate orhydroxy carbonate ions. The molar ratio of hydroxide ion to total metalions can vary widely. In various embodiments the molar ratio ofhydroxide to metals ranges from about 1:1 to about 2:1, to as high asabout 5:1. In various other embodiments the amount of hydroxide ion islimited to maintain a reaction pH of less than about 7, less than about5, and less than about 4.5. In another embodiment, the amount ofhydroxide is a quantity sufficient to form nanoparticles.

The nanoparticle stabilizers of the invention are monoether carboxylicacids. In one embodiment, the nanoparticle stabilizer is water-solubleand forms weak bonds with the cerium ion. In another embodiment, thenanoparticle stabilizer is a monoether carboxylic acid of formula (I).

ROCHR¹CO₂Y  (1)

In formula (I), R represents a substituted or unsubstituted alkyl group(C₁-C₄), for example, a methyl group, an ethyl group; or an aromaticgroup such as a phenyl group. R¹ represents hydrogen or a substituentgroup such as an alkyl group. In formula (1), Y represents H or acounterion such as an alkali metal ion, for example, Na⁺ or K⁺. Anon-limiting list of monoether carboxylic acids includes: ethoxyaceticacid, methoxyacetic acid, 3-methoxypropionic acid, and combinationsthereof. The monoether carboxylic acid stabilizers are present in anamount such that the molar ratio of stabilizer to the total metal ions(or metal oxides) is greater than 0.2. In various other embodiments themolar ratio of stabilizer to the total metal ions (or metal oxides) isgreater than 0.25, greater than 0.3, and greater than 0.6. While notwishing to be held to any particular theory, the carboxylic acid groupmay bind to the nanoparticle surface, while the remainder of thestabilizer (i.e. ether moiety) prevents agglomeration of the particlesand the subsequent formation of large clumps of particles.

In a particular embodiment, the cerium oxide or doped cerium oxidenanoparticles are formed in an aqueous environment and combined with oneor more nanoparticle stabilizers. In other embodiments, the cerium oxideor doped cerium oxide nanoparticles are formed in the presence of thestabilizer(s), formed at least in part in the presence of thestabilizer(s), or the stabilizer(s) is added shortly after theirformation.

In other embodiments the nanoparticles are synthesized in solvents orsolvent mixtures that are less polar than water. Regardless of whetherthe synthesized nanoparticles are made in a hydrophilic or hydrophobicmedium, however, dispersions of cerium-containing nanoparticles benefitsubstantially from a stabilizer additive of the invention in regard toreducing undesirable agglomeration. Additionally, the amount of thestabilizer in the reaction mixture is critical when an oxidant morepowerful than ambient air is also present.

In various embodiments, oxidants for use in the invention includecompounds more oxidizing than molecular oxygen (or an ambient atmosphereof air). In electrochemical half cell reaction terms, suitable oxidantsare compounds with a aqueous half cell reduction potential greater than−0.13 volts relative to a standard hydrogen electrode. In particularembodiments the oxidant is an alkali metal or ammonium perchlorate,chlorate, hypochlorite, or persulfate; ozone, or hydrogen peroxide, orcombinations thereof. The amount of oxidant in relation to the amount ofmetal ions to be oxidized can vary widely. In particular embodiments themolar equivalent amount of oxidant present is equal to or greater thanthe total molar equivalent amount of metal ions to be oxidized. Inspecific embodiments, two-electron oxidants, such as hydrogen peroxide,are present in at least one-half the molar concentration of the ceriumion.

In some embodiments the cerium-containing oxide nanoparticles exhibit anX-ray diffraction pattern characteristic of the cubic fluoritestructure.

Cerium-containing nanoparticles can be prepared by a variety oftechniques known in the art. Some of these synthetic techniques aredescribed in the following publications: U.S. Pat. Nos. 6,271,269;6,649,156; 7,008,965; U.S. Patent Appl. Publ. Nos. 2004/0029978(abandoned Dec. 7, 2005); 2006/0005465; U.S. Pat. No. 7,025,943; WO2008/002223 A2; U.S. Pat. No. 4,231,893; U.S. Patent Appl. Publ. Nos.2004/0241070; 2005/0031517; U.S. Pat. Nos. 6,413,489; 6,869,584; U.S.Patent Appl. Publ. No. 2005/0152832; U.S. Pat. No. 5,938,837; EuropeanPatent Application EP 0208580, published 14 Jan. 1987; U.S. Pat. Nos.7,419,516; and 6,133,194.

As described above, crystalline cerium dioxide nanoparticles can beprepared by various procedures. In some embodiments, the syntheticroutes utilize water as a solvent and yield an aqueous mixture ofnanoparticles and one or more salts. For example, cerium dioxideparticles can be prepared by reacting the hydrate of cerium (III)nitrate with hydroxide ion from, for example, aqueous ammoniumhydroxide, and thereby forming cerium (III) hydroxide, as shown inequation (2a). Cerium hydroxide can be oxidized to cerium (IV) dioxidewith an oxidant such as hydrogen peroxide, as shown in equation (2b).The analogous tris hydroxide stoichiometry is shown in equations (3a)and (3b).

Ce(NO₃)₃(6H₂O)+2NH₄OH→Ce(OH)₂NO₃+2 NH₄NO₃+6H₂O  (2a)

2Ce(OH)₂NO₃+H₂O₂→2 CeO₂+2 HNO₃+2H₂O  (2b)

Ce(NO₃)₃(6H₂O)+3NH₄OH→Ce(OH)₃+3 NH₄NO₃+6H₂O  (3a)

2Ce(OH)₃+H₂O₂→2 CeO₂+4H₂O  (3b)

Complexes formed with very high base levels, e.g. 5 to 1 ratio of OH toCe, also provide a route to cerium oxide, albeit at much larger grainsizes if not properly growth-restrained.

In some cases, especially those in which ammonium hydroxide is notpresent in excess relative to the cerous ion, the species Ce(OH)₂(NO₃)or (NH₄)₂Ce(NO₃)₅ may initially be present, subsequently undergoingoxidation to cerium dioxide.

Commonly assigned PCT/US2007/077545, METHOD OF PREPARING CERIUM DIOXIDENANOPARTICLES, filed Sep. 4, 2007, describes a mixing device that iscapable of producing CeO₂ nanoparticles down to 1.5 nm, in high yieldand in very high suspension densities. The reactor includes inlet portsfor adding reactants, a propeller, a shaft, and a motor for mixing. Moreparticularly, in one embodiment, a high shear mixer such as a colloidmill manufactured by Silverson Machines, Inc. is employed to agitate thereaction mixture.

In another embodiment, the present invention provides for a continuousprocess for producing cerium-containing oxide nanoparticles, optionallycontaining one or more transition and/or rare earth metal ions, having amean hydrodynamic diameter of about 1 nm to about 50 nm, wherein theprocess comprises the steps of combining cerous ion, optionally one ormore metal ions other than cerium, an oxidant, at least one monoethercarboxylic acid nanoparticle stabilizer, wherein the molar ratio of themonoether carboxylic acid nanoparticle stabilizers to total metal ionsis greater than 0.2, and hydroxide ion within a continuous reactor. In aparticular embodiment, the cerium-containing oxide nanoparticlesproduced by a continuous process are crystalline.

When an aqueous preparation is employed, the cerium oxide nanoparticledispersion is typically purified, wherein the unreacted cerium salts(e.g. nitrate) and waste by-products (e.g. ammonium nitrate) areremoved, most conveniently, for example, by diafiltration (transverseflow filtration through a semi-permeable membrane). In order to promotesubsequent solvent shifting into less polar media, including non-polarmedia, it is desirable to reduce the ionic strength to a conductivity ofabout 5-10 mS/cm or less. Alternatively, the nanoparticles may bepurified by other means, for example, by centrifugation. The productdispersion may be diluted or concentrated before, during, or after thepurification process.

In another embodiment, a process is provided for forming a homogeneousdispersion containing undoped or homogeneously doped cerium oxidenanoparticles, at least one monoether carboxylic acid nanoparticlestabilizer, a solvent less polar than water, at least one surfactant,and a non-polar medium. In particular embodiments, glycol ether solventsof a polarity intermediate between that of water and those of non-polarhydrocarbons are used to reduce the polarity of the nanoparticledispersion, as disclosed in commonly assigned U.S. patent applicationSer. No. 12/549,776, PROCESS FOR SOLVENT SHIFTING A NANOPARTICLEDISPERSION, filed Aug. 28, 2009. Specific embodiments employ diethyleneglycol monomethyl ether, 1-methoxy-2-propanol, or a mixture thereof, aspolarity shift solvents. In particular embodiments, the water content ofthe nanoparticle dispersions are reduced to less than about 10 wt. %,less about 5 wt. %, less than about 2 wt. %, and less than 0.5 wt. %. Inparticular embodiments, the cerium-containing oxide nanoparticle contentof the dispersion is increased (concentrated) to about 35-40 wt. %.

In another embodiment, the undoped or homogeneously doped cerium oxidenanoparticles dispersed in an intermediate polarity medium of low watercontent, still stabilized in part by the original monoether carboxylicacid stabilizer, and by a glycol ether shift solvent, are subsequentlydispersed homogeneously into a higher molecular weight surfactant, suchas oleic acid, which in turn is soluble in non-polar hydrocarbondiluents, such as kerosene, which is compatible with most hydrocarbonfuels such as diesel and biodiesel. In one embodiment, the oleic acidalso contains a co-surfactant such as 1-hexanol. While not wishing to beheld to any particular theory, it is important to realize that thiscomposition of matter is not a reverse micelle water-in-oil emulsion, asthere is very little water present; rather, the positive charge on thesurface of the cerium nanoparticle has been complexed by the etheroxygen atoms and bound to the oppositely charged carboxylic acid. Thehigher molecular weight carboxylic acid surfactant (e.g. oleic acid) ispresent in a chemisorbed or physisorbed state and facilitates themiscibility of the nanoparticle with a non-polar hydrocarbon diluent. Insome embodiments, the higher molecular weight surfactants are carboxylicacids with carbon chain lengths less than about 20 carbon atoms butgreater than about 8 carbon atoms. In particular embodiments, highermolecular weight surfactants such as linoleic acid, stearic acid, andpalmitic acid are used in place of oleic acid.

In another embodiment, a process is provided for combining thehomogeneous dispersion containing undoped or homogeneously doped ceriumoxide nanoparticles in the higher molecular weight surfactant, with anon-polar hydrocarbon diluent to form a fuel additive concentrate. Invarious embodiments the non-polar hydrocarbon diluent is a hydrocarboncontaining about 6-20 carbon atoms, including, for example, aliphatichydrocarbons such as hexane, heptane, octane, nonane, decane; inertcycloaliphatic hydrocarbons such as cyclopentane, cyclohexane, orcycloheptane; aromatic hydrocarbons such as benzene, toluene,ethylbenzene, xylenes or liquid naphthenes; kerosene (e.g. KENSOL® K1),naphtha, diesel fuel, biodiesel, gasoline, petroleum distillates, (e.g.SOLVESSO® 100 and SOLVESSO® 150), hydrotreated petroleum distillates(e.g. KENSOL® 48H, KENSOL® 50H) and paraffin oils (e.g. offerings underthe ISOPAR® tradename), and mixtures thereof. In particular embodiments,additional glycol ether shift solvent (e.g. diethylene glycol monomethylether, 1-methoxy-2-propanol, or a mixture thereof) is added to the fueladditive concentrate such that the total glycol ether content is about5%, about 10%, about 15%, or about 20% by volume. In one embodiment,when used as a fuel additive, one part of the fuel additive concentrateis combined with at least about 100 parts of the fuel.

Dispersions of undoped or homogeneously doped cerium oxide nanoparticlesof the invention can be used in many applications. By way of example,the following publications describe fuel additives containing ceriumoxidic compounds: U.S. Pat. Nos. 5,449,387; 7,063,729; 6,210,451;6,136,048; 6,093,223; 7,195,653 B2; U.S. Patent Appl. Publ. Nos.2003/0182848; 2003/0221362; 2004/0035045; 2005/0060929; 2006/0000140;International Publ. Nos. WO 2004/065529; and WO 2005/012465.

As is known to those skilled in the chemical arts, cerium oxide orcerium dioxide is widely used as a catalyst in automotive catalyticconverters for the elimination of toxic exhaust emission gases and indiesel particulate filters for the reduction of particulate emissions indiesel powered vehicles. Within the catalytic converter or dieselparticulate filter, cerium-containing oxide particles can act as achemically active component, acting to release oxygen in the presence ofreductive gases, as well as to remove oxygen by interaction withoxidizing species. The chemical reactivity of a fully oxidized ceriabased three-way or fuel borne catalyst can conveniently be measured byfollowing the progress of the reaction:

CeO₂→CeO_(2-w)+w/2O₂

The extent to which the reaction gives off oxygen (the number of molesof O₂) is called the oxygen storage capacity (OSC) and the rate at whichthis happens is embodied in the rate constant, k. Alternatively, the OSCof a reduced form of the three-way or fuel borne catalyst canconveniently be measured by following the progress of the reversereaction. It is understood that high OSC and high k, are associated withhigher reactivity catalysts. The undoped and homogeneously doped ceriumoxide nanoparticles of the present invention and the processes formaking thereof, can be used to form catalysts for these purposes.

Motor oil is used as a lubricant in various kinds of internal combustionengines in automobiles and other vehicles, boats, lawn mowers, trains,airplanes, etc. Engines contain contacting parts that move against eachother at high speeds, often for prolonged periods of time. Those movingparts create friction, forming a temporary weld, immobilizing the movingparts. Breaking this temporary weld absorbs otherwise useful powerproduced by the motor and converts the energy to useless heat. Frictionalso wears away the contacting surfaces of those parts, which may leadto increased fuel consumption and lower efficiency and degradation ofthe motor. In one aspect of the invention, a motor oil includes alubricating oil and undoped or metal-containing, crystalline,homogeneously doped cerium oxide nanoparticles, having a mean diameterof 1.5-10 nm, or a mean diameter of 2-4 nm, or alternatively, a meandiameter of 2-3 nm, and optionally a surface adsorbed stabilizing agentthat is delivered as a homogeneous dispersion in a non-polar medium.

Diesel lubricating oil and fuels are essentially free of water(preferably less than 300 parts per million (ppm)) but may be modifiedby the addition of an undoped or homogeneously doped cerium oxidenanoparticle composition, wherein these compositions have been solventshifted from their aqueous reaction environment to that of an organic ornon-polar medium. In particular embodiments, these undoped orhomogeneously doped cerium oxide compositions include nanoparticleshaving a mean diameter of less than about 6 nm, or less than about 4 nm,or less than about 3 nm, as already described. A diesel engine operatedwith modified diesel fuel and/or modified lubricating oil providesgreater efficiency and may, in particular, provide improved fuelmileage, reduced engine wear or reduced pollution, or a combination ofthese features.

Metal polishing, also termed buffing, is the process of smoothing metalsand alloys and polishing to a bright, smooth mirror-like finish. Metalpolishing is often used to enhance cars, motorbikes, antiques, etc. Manymedical instruments are also polished to prevent contamination inirregularities in the metal surface. Polishing agents are also used topolish optical elements such as lenses and mirrors to a surfacesmoothness within a fraction of the wavelength of the light they are tomanage. Polishing agents may be used for planarization (rendering thesurface smooth at the atomic level) of semiconductor substrates forsubsequent processing of integrated circuits. Homogeneous dispersions ofuniformly dimensioned undoped or metal-containing homogeneously dopedcerium oxide particles in aqueous media of varying acidity/alkalinity,in media of reduced polarity relative to water, or in non-polar media,may be advantageously employed as polishing agents in polishingoperations.

The invention is further illustrated by the following examples, whichare not intended to limit the invention in any manner.

EXAMPLES Particle Size Measurement Hydrodynamic Diameter

Characterization of the particle size of an aqueous dispersion wasprovided using a Brookhaven 90Plus Particle Size Analyzer (BrookhavenInstruments Corp., Holtzville, N.Y., U.S.A.), which determines thehydrodynamic diameter of the particles by dynamic light scattering (DLS)techniques. Reported sizes are the lognormal number weighted parameter.

Oxygen Storage Capacity Measurement

Aqueous sols of cerium-containing nanoparticles prepared as describedherein were heated for 30 minutes in a muffle furnace at 1000° C. toremove the organic stabilizer, then ground to a uniform consistency toremove any effects from mild sintering. These thoroughly dried sampleswere measured for OSC and the kinetics at which they reached theirmaximum OSC using thermogravimetric techniques. More specifically, OSCwas measured using a TA Instruments Q500 thermo-gravimetric analyzer(TGA). The thoroughly dried/ground samples were then heated in the TGAfurnace to 700° C. under air and allowed to stay at that temperature for15 minutes. The sample was exposed to a reducing environment consistingof 5% H₂ in nitrogen for 40 min. Then, the sample was exposed to air for15 min. This was all completed at 700° C. The weight change was recordedby the TA instrument. The OSC calculation used is: (Final weight underOxygen−Final weight under H₂/N₂)/(32× Sample Weight) and the measuredvalues are reported as μ moles O₂/g sample.

In some embodiments, one observes a very fast initial reduction rate(i.e. sample weight loss) in nitrogen gas containing 5% hydrogen,followed by a second slower rate. The accompanying Table 2 contains theOxygen Storage Capacity (1 sigma reproducibility in parenthesis) and thefast (k₁) and slow (k₂) rate constants (1 standard deviation inparenthesis) for reduction of various cerium, iron and zirconiumcontaining nanoparticles (all about 2 nm except the Sigma Aldrichcontrol) in a nitrogen gas at 700° C. containing 5% H₂. These valueshave been cross-checked against a second TGA instrument (average 2.6%difference), against gas flow differences (average 1% deviation) andreplicate sample preparation at 1000° C. for 30 minutes (average 1.54%deviation).

Background to Examples 1-2: These examples are an attempt to combine twodistinct teachings from earlier work, ones taken from Cuif et al., U.S.Pat. No. 6,133,194, and Poncelet et al., FR 2885308. Once more, Cuif etal. (U.S. Pat. No. 6,133,194) describes many additives other thanmonoether carboxylic acids for use in conventional aqueous precipitationprocesses for the preparation of cerium oxides, zirconium oxides,cerium/zirconium mixed oxides and cerium/zirconium solid solutions. Itis also suggested therein, but not exemplified, that optionally anoxidizing agent can be used, preferably hydrogen peroxide, which can beadded to the cerium/zirconium mixture or to the cerium or zirconium saltbefore the metals are mixed together. It is suggested that the amount ofoxidizing agent in relation to the metal salts to be oxidized 1) canvary within wide limits, and 2) is generally greater than astoichiometric amount, corresponding to an excess. Poncelet et al. (FR2885308) describes the use of alkoxy carboxylic acids (both monoethercarboxylic acids and polyether carboxylic acids) as additives in anaqueous precipitation processes of Cuif et al. (U.S. Pat. No.6,133,194), and without the addition of an oxidant, such as hydrogenperoxide, provides a direct process (not requiring the steps ofprecipitation, isolation and calcination) of obtaining dispersions(colloidal solutions) of cerium oxide particles having aggregates thatare much finer and better dispersed than the cerium oxide obtained witheither sodium dioctyl-sulfo-succinate additive or EDTA additive, orwithout an additive. Poncelet et al. (FR 2885308) clearly specifies thatthe alkoxy carboxylic acid/metallic oxide molar ratio is between 0.01and 0.2. It is further specified that preferably the alkoxy carboxylicacid/metallic oxide molar ratio is between 0.05 and 0.15. Example 3 ofPoncelet et al. (FR 2885308) describes the use of a monoether carboxylicacid (3-methoxypropionic acid) in an aqueous synthesis process of Cuifet al. (U.S. Pat. No. 6,133,194) to produce aggregates of cerium oxideparticles with a hydrodynamic diameter of 50-60 nm.

Examples 1-2 combine the suggestion of Cuif et al. (U.S. Pat. No.6,133,194) to use an oxidizing agent, hydrogen peroxide, in a wide rangeof amounts (herein 0.2, 1.0 and 2.0 times a stoichiometric amountrelative to the metals (e.g. cerous ion) available to be oxidized), withthe teachings of Poncelet et al. (FR 2885308) that an alkoxy carboxylicacid (e.g. monoether carboxylic acid) additive used in the nanoparticlepreparation process of Cuif et al. (U.S. Pat. No. 6,133,194) must be inan amount such that the alkoxy carboxylic acid/metallic oxide molarratio is in the range of 0.01 and 0.2, and preferably in the range of0.05 to 0.15.

Example 1a Preparation of CeO_(2-δ) using 0.125 MAA/Total Metals MolarRatio with 1× Hydrogen Peroxide Oxidant (comparative)

The procedures of Example 1 of Poncelet et al. (FR 2885308) wererepeated, except that an equimolar amount of MAA (methoxyacetic acid)was used in place of MEAA (2-(2-methoxyethoxy)acetic acid), and astoichiometric amount of hydrogen peroxide (H₂O₂) oxidant relative tothe amount of cerium salt was added to the cerium salt solution. Themolar ratio of MAA to total metals was 0.125. The hydrodynamic diameterof the resulting suspension of cerium oxide particles was 188 nm.

Example 1b Preparation of CeO_(2-δ) using 0.125 MAA/Total Metals MolarRatio with 2× Hydrogen Peroxide Oxidant (comparative)

The procedures of Example 1a were repeated except that the amount ofhydrogen peroxide was increased to 2× a stoichiometric amount relativeto the amount of cerium salt. The hydrodynamic diameter of the resultingsuspension of cerium oxide particles was 202 nm.

Example 2a Preparation of Ce_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) using 0.125MAA/Total Metals Molar Ratio with 0.2× Hydrogen Peroxide Oxidant(comparative)

The procedures of Examples 1 of Poncelet et al. (FR 2885308) wererepeated, except that an equimolar amount of MAA (methoxyacetic acid)was used in place of MEAA (2-(2-methoxyethoxy)acetic acid), andappropriate amounts of Ce(NO₃)₃.6H₂O, ZrO(NO₃)₂.xH₂O and Fe(NO₃)₃.9H₂Oto form Ce_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) were added to the metal saltsolution (homogeneous doping). The hydrodynamic diameter of theresulting suspension of cerium-zirconium-iron containing oxide particleswas 1110 nm.

Example 2b Preparation of Ce_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) using 0.125MAA/Total Metals Molar Ratio with 0.2× Hydrogen Peroxide Oxidant(comparative)

The procedures of Example 2a were repeated, except that 0.2× astoichiometric amount of hydrogen peroxide (H₂O₂) oxidant relative tothe total amount of metal salts was added to the metal salt solution.The hydrodynamic diameter of the resulting suspension ofcerium-zirconium-iron containing oxide particles was 1253 nm.

Example 2c Preparation of Ce_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) using 0.125MAA/Total Metals Molar Ratio with 1× Hydrogen Peroxide Oxidant(comparative)

The procedures of Example 2a were repeated, except that a stoichiometricamount of hydrogen peroxide (H₂O₂) oxidant relative to the total amountof metal salts was added to the metal salt solution. The hydrodynamicdiameter of the resulting suspension of cerium-zirconium-iron containingoxide particles was 698 nm.

Example 2d Preparation of Ce_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) using 0.125MAA/Total Metals Molar Ratio with 2× Hydrogen Peroxide Oxidant(comparative)

The procedures of Example 2a were repeated, except that 2× astoichiometric amount of hydrogen peroxide (H₂O₂) oxidant relative tothe total amount of metal salts was added to the metal salt solution.The hydrodynamic diameter of the resulting suspension ofcerium-zirconium-iron containing oxide particles was 1309 nm.

A summary of the particle sizes for Examples 1-2 are shown in Table 1.

TABLE 1 Particle Size Results for H₂O₂ Oxidant Level Series Stabilizer/Total Metals Particle Oxidant Molar Size Ex. Composition Oxidant AmountStabilizer Ratio (DLS) Comment 1a CeO_(2-δ) H₂O₂ 1x MAA 0.125  188 nmComparative 1b CeO_(2-δ) H₂O₂ 2x MAA 0.125  202 nm Comparative 2aCe_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) — — MAA 0.125 1110 nm Comparative 2bCe_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) H₂O₂   0.2x MAA 0.125 1253 nmComparative 2c Ce_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) H₂O₂ 1x MAA 0.125  698nm Comparative 2d Ce_(0.75)Zr_(0.15)Fe_(0.1)O_(2-δ) H₂O₂ 2x MAA 0.1251309 nm Comparative

The results shown above for Examples 1a-1b indicate that the particlesizes of CeO_(2-δ) produced using a 0.125 MAA/total metals molar ratioand 1× and 2× hydrogen peroxide oxidant (188 nm and 202 nm,respectively) are substantially larger than those reported earlier(Example 3 in Poncelet et al. (FR 2885308)), wherein use of a comparableamount of the monoether carboxylic acid (3-methoxypropionic acid)without H₂O₂ oxidant produced cerium oxide particles with a hydrodynamicdiameter of 50-60 nm. Results for Examples 2a-2d, wherein a level seriesin H₂O₂ consisting of none, 0.2×, 1× and 2× of a stoichiometric amountof the total metal salts was used along with a preferred level ofmonoether carboxylic acid additive (molar ratio of 0.125 MAA/totalmetals) according to the disclosure of Poncelet et al., and cerium,zirconium and iron as metal salts, show very poor particle sizes(698-1309 nm) relative to the previously taught levels of 50-60 nm(Example 3 in Poncelet et al. (FR 2885308)). Therefore we conclude thatcombining the suggestion in Cuif et al. (U.S. Pat. No. 6,133,194) to adda wide range of an oxidant, such as hydrogen peroxide, to the earlierlimitations taught by Poncelet et al. (FR 2885308), namely that themolar ratio of monoether carboxylic acid additive relative to totalmetal ions must be in the range 0.01-0.2, fails to reduce the particlesize further below that achieved without the oxidant.

Example 3 explored the use of hydrogen peroxide additive along withamounts of a monoether carboxylic acid additive well above the limitspecified by Poncelet et al. (FR 2885308). A series of homogeneouslyiron doped cerium oxide nanoparticle preparations,Ce_(0.6)Fe_(0.4)O_(2-δ), using H₂O₂ oxidant and MAA stabilizer, whereinthe molar ratio of MAA to total metal ions was increased from 0.155 to2.48, were conducted.

Example 3a Preparation of Ce_(0.6)Fe_(0.4)O_(2-δ) using H₂O₂ oxidantwith 0.155 MAA/Total Metals Molar Ratio (comparative)

To a 400 ml glass beaker containing a one inch magnetic stir bar, 1.50grams of (98%) methoxyacetic acid (MAA) and 84.7 ml of distilled waterwere introduced. The beaker was then placed into a water bath at atemperature of about 75° C. with constant bar stirring. A metal saltsolution containing 27.0 grams of cerium (III) nitrate hexahydrate and17.3 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 40 grams distilled water, was drawn into a syringe pump andthen subsequently pumped at a rate of about 12 ml/minute into the MAAcontaining beaker through a subsurface jet positioned close to the stirbar. Concurrent with the start of the metal salt solution addition, analiquot of about 26 ml of concentrated (28-30%) ammonium hydroxide waspumped into the reaction vessel in a similar fashion at a rate of about6 ml/minute. The actual amount of ammonium hydroxide to be delivered isdependent on the desired pH of the reaction Ammonium hydroxide was addeduntil a pH of 5 was achieved at which time the addition was stopped. A8.3 ml aqueous solution containing 7.9 grams of 50 wt. % hydrogenperoxide was then pumped into the reaction beaker at a rate of 0.83ml/minute via a syringe pump. When all reagents had been added, thereaction mixture was an opaque dark orange brownish color containing asubstantial amount of precipitate, at about a pH of 5. The reactionmixture was then heated with stirring for an additional 60 minutes at75-80° C. degrees, during which time the pH dropped to about 3.9. Uponcooling the reaction mixture formed orange brown sediment that occupiedthe lower ⅔ of the reaction vessel, above which a clear yellow orangesupernatant resided. Particle size analysis of the supernatantdispersion by dynamic light scattering indicated a hydrodynamic diameterof about 226 nm, whereas the sediment had a hydrodynamic diameter ofabout 721 nm.

Example 3b Preparation of Ce_(0.60)Fe_(0.40)O_(2-δ) using H₂O₂ oxidantwith 0.31 MAA/Total Metals Molar Ratio (inventive)

The procedures of Example 3a were repeated, except the amount of (98%)methoxyacetic acid (MAA) stabilizer was increased to 3.0 grams, suchthat a molar ratio of MAA to total metal ions of 0.31 was achieved. Whenall reagents had been added, the reaction mixture was an opaque darkorange brownish color at about a pH of 5. The reaction mixture was thenheated with stirring for an additional 60 minutes at 75-80° C. degrees,during which time the pH dropped to about 3.9, and the mixture became aclear yellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.Particle size analysis of the clear yellow orange dispersion by dynamiclight scattering indicated a hydrodynamic diameter of about 50.2 nm.

Example 3c Preparation of Ce_(0.60)Fe_(0.40)O_(2-δ) using H₂O₂ oxidantwith 0.62 MAA/Total Metals Molar Ratio (inventive)

The procedures of Example 3a were repeated, except the amount of (98%)methoxyacetic acid (MAA) stabilizer was increased to 6.0 grams, suchthat a molar ratio of MAA to total metal ions of 0.62 was achieved. Whenall reagents had been added, the reaction mixture was an opaque darkorange brownish color at about a pH of 5. The reaction mixture was thenheated with stirring for an additional 60 minutes at 75-80° C. degrees,during which time the pH dropped to about 3.9, and the mixture became aclear yellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.Particle size analysis of the clear yellow orange dispersion by dynamiclight scattering indicated a hydrodynamic diameter of about 6.9 nm.

Example 3d Preparation of Ce_(0.60)Fe_(0.40)O_(2-δ) using H₂O₂ oxidantwith 1.24 MAA/Total Metals Molar Ratio (inventive)

The procedures of Example 3a were repeated, except the amount of (98%)methoxyacetic acid (MAA) stabilizer was increased to 12.0 grams, suchthat a molar ratio of MAA to total metal ions of 1.24 was achieved. Whenall reagents had been added, the reaction mixture was an opaque darkorange brownish color at about a pH of 5. The reaction mixture was thenheated with stirring for an additional 60 minutes at 75-80° C. degrees,during which time the pH dropped to about 3.9, and the mixture became aclear yellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.Particle size analysis of the clear yellow orange dispersion by dynamiclight scattering indicated a hydrodynamic diameter of about 7.6 nm.

Example 3e Preparation of Ce_(0.60)Fe_(0.40)O_(2-δ) using H₂O₂ oxidantwith 2.48 MAA/Total Metals Molar Ratio (inventive)

The procedures of Example 3a were repeated, except the amount of (98%)methoxyacetic acid (MAA) stabilizer was increased to 24.0 grams, suchthat a molar ratio of MAA to total metal ions of 2.48 was achieved. Whenall reagents had been added, the reaction mixture was an opaque darkorange brownish color at about a pH of 5. The reaction mixture was thenheated with stirring for an additional 60 minutes at 75-80° C. degrees,during which time the pH dropped to about 3.9, and the mixture became aclear yellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.Particle size analysis of the clear yellow orange dispersion by dynamiclight scattering indicated a hydrodynamic diameter of about 6.2 nm.

Table 2 lists the hydrodynamic diameter (DLS) particle size results forthe cerium and iron containing particles of Example 3, which wereprepared with H₂O₂ oxidant and various amounts of MAA stabilizer.

TABLE 2 Size Results for use of H₂O₂ oxidant with MAA Stabilizer LevelSeries Stabilizer/Total Particle Composition Metals Size Ex. (Method 1)Oxidant Stabilizer Molar Ratio (DLS) Comment 3a Ce_(0.6)Fe_(0.4)O_(2-δ)H₂O₂ MAA 0.155 226.0 nm  Comparative 3b Ce_(0.6)Fe_(0.4)O_(2-δ) H₂O₂ MAA0.31 50.2 nm  Inventive 3c Ce_(0.6)Fe_(0.4)O_(2-δ) H₂O₂ MAA 0.62 6.9 nmInventive 3d Ce_(0.6)Fe_(0.4)O_(2-δ) H₂O₂ MAA 1.24 7.4 nm Inventive 3eCe_(0.6)Fe_(0.4)O_(2-δ) H₂O₂ MAA 2.48 6.2 nm Inventive

Comparison of the particle size (DLS) results shown above for Example 3ato Examples 3b-3d indicates that a surprising and dramatic decrease inparticle size when the molar ratio of methoxyacetic acid stabilizer(MAA) to total metals is greater than 0.2, for homogeneously iron dopedcerium-containing oxide nanoparticles prepared using hydrogen peroxideoxidant. Results of OSC measurements also showed an increase withincreased MAA stabilizer level.

Examples 4-8 compare a commercially obtained ceria nanoparticle sampleto undoped and homogeneously doped cerium-containing nanoparticlesprepared by a process (Method 1) comprising the sequential steps: 1)providing an aqueous solution of a methoxyacetic acid stabilizer, 2)concurrent addition of metal salts along with addition of hydroxide ion,3) addition of hydrogen peroxide oxidant, wherein the molar ratio ofmethoxyacetic acid stabilizer to total metals was 2.35.

Example 4a Commercial Cerium(IV) Oxide Nanopowder

Cerium(IV) oxide nanopowder, <25 nm particle size (BET) was purchasedfrom Sigma-Aldrich® and characterized as received by TEM and OSC.However, examination by TEM revealed a highly aggregated material withchunks on the order of 20 microns. This is consistent with theinstructions received from the vendor that the material could be ballmilled by the end user to a size less than 25 nm.

Example 4b Preparation of CeO_(2-δ)Nanoparticles by IsothermalDouble-Jet Precipitation

To a 3 liter round bottom stainless steel reactor vessel was added 1117grams of distilled water. An impeller (Lightnin® R-100 Rushton styleturbine) was lowered into the reactor vessel, and the mixer head waspositioned slightly above the bottom of the reactor vessel. The mixerwas set to 700 rpm, and the reactor was brought to a temperature ofabout 70° C. Then 59.8 grams (98%) of methoxyacetic acid were added tothe reactor. A double jet precipitation was conducted over a period offive minutes by pumping a 250 ml solution containing 120.0 grams ofCe(NO₃)₃.6H₂O into the reactor concurrently with a solution containing69.5 grams (28-30%) of ammonium hydroxide. A distilled water chase intothe reactor cleared the reactant lines of residual materials. Then 10.2grams of 50% non-stabilized hydrogen peroxide was added to the reactorand its contents over a period of 40 seconds. Initially, the reactionmixture was an opaque dark orange brownish liquid in the pH range 6 to7. The reaction mixture was heated for an additional 60 minutes, duringwhich time the pH dropped to 4.25 (consistent with the release ofhydronium ion via reactions (3a) and (3b) and the mixture became clearyellow orange color. The reaction was cooled to 20° C. and diafilteredto a conductivity of 3 mS/cm to remove excess water and unreactedmaterials. This resulted in concentrating the dispersion by a factor ofabout 10, or nominally 1 Molar in CeO₂ particles. FIG. 1A is a highmagnification TEM of a dispersion of particles of Example 5, from whicha particle size-frequency analysis (FIG. 1B) revealed a mean particlesize of 2.2±0.5 nm, with size frequency distribution having acoefficient of variation, COV, (one standard deviation divided by themean diameter) of 23%. The calculated yield was 62.9%.

Example 5 Iron-Containing CeO₂ Nanoparticles: Ce_(0.9)Fe_(0.1)O_(2-δ)

The conditions of Example 4 were repeated, except that the metal saltssolution contained 108.0 grams of cerium nitrate hexahydrate, and 11.16grams of Fe(NO₃)₃.9H₂O. These metal salts were dissolved separately andthen combined to form a 250 ml solution. The reaction proceeded asdescribed in Example 5. FIG. 2A is a high magnification TEM of thedispersed particles, from which a particle size-frequency analysis (FIG.2B) revealed a mean particle size of 2.2±0.7 nm, with size frequencydistribution having a coefficient of variation, COV, (one standarddeviation divided by the mean diameter) of 32%. The calculated yield was55.1%.

Example 6 Zirconium-Containing CeO₂ Nanoparticles:Ce_(0.85)Zr_(0.15)O_(2-δ)

The conditions of Example 4 were repeated except that the metal saltssolution contained 101.89 grams of cerium nitrate hexahydrate, and 9.57grams of ZrO(NO₃)₂.xH₂O. These metal salts were dissolved separately andthen combined to form a 250 ml solution. The reaction proceeded asdescribed in Example 5, except that the temperature of the reaction wascarried out at 85° C. Particle size-frequency analysis by transmissionelectron micrography (FIGS. 3A and 3B) revealed a mean particle size of2.4±0.7 nm, with size frequency distribution having a coefficient ofvariation, COV, (one standard deviation divided by the mean diameter) of29%. Inductively coupled plasma atomic emission spectroscopy revealed astoichiometry of Ce_(0.82)Z_(0.18)O_(1.91), which given the relativeinsolubility of ZrO₂ to CeO₂, would account for the enhanced Zr content(18% vs 15%).

Example 7a Zirconium and Iron Containing CeO₂ Nanoparticles:Ce_(0.75)Zr_(0.15)Fe_(0.10)O_(2-δ)

The conditions of Example 44 were repeated, except that the metal saltssolution contained 84.0 grams of cerium nitrate hexahydrate, 11.16 gramsof Fe(NO₃)₃. 9 H₂O and 12.76 grams of ZrO(NO₃)₂.xH₂O. These metal saltswere dissolved separately and then combined to form a 250 ml solution.The reaction proceeded as described in Example 4, except that thetemperature of the reaction was carried out at 85° C., and the hydrogenperoxide solution (50%) was elevated to 20.4 gm and added over a periodof ten minutes. Particle TEM (FIG. 4A) and particle size-frequencyanalysis by transmission electron micrography (FIG. 4B) revealed a meanparticle size of 2.2±0.6 nm, with size frequency distribution having acoefficient of variation, COV, (one standard deviation divided by themean diameter) of 27%. A monodisperse, unimodal distribution supportsthe idea of co-incorporation of zirconium and iron ions into a ceriahost particle, however, separately nucleated or renucleated zirconiarich and/or iron oxide rich grain populations, for example, may also bepresent. The calculated yield was 78%. Inductively coupled plasma atomicemission spectroscopy revealed a stoichiometry ofCe_((0.69))Fe_((0.14))Zr_((0.17))O_((1.915)). Again, the relatively moreconcentrated Fe and Zr with respect to the nominal amounts may reflectthe greater insolubility of their hydroxide precursors relative to thatof cerium hydroxide. Also in FIG. 4C is an x-ray powder diffractionpattern of this sample (top curve) compared to the transition metal freeCeO₂. The lack of a peak (denoted by an arrow) at 32 deg two theta meansthat there is no evidence of free ZrO₂, i.e., it may be fullyincorporated into the cerium lattice. Also, the lack of peaks at 50 and52 degrees two theta indicate no evidence of a separate population ofFe₂O₃ (i.e. consistent with incorporation of Fe into the ceriumlattice). Note the shift to larger two theta at large two thetascattering angle, which indicates a distortion or contraction of thelattice-(n λ/2d=sin θ), is consistent with the smaller ionic radii ofFe³⁺(0.78A) and Zr⁴⁺ (0.84A) relative to the Ce⁴⁺ (0.97A) which it isreplacing. Thus, we note evidence that the transition metals zirconiumand iron may be incorporated into the CeO₂ lattice, although separatepopulations of poorly crystalline or amorphous nanoparticles of, forexample, zirconium-rich oxides and/or iron-rich oxides may also bepresent.

Examples 7b-f Zirconium and Iron Containing CeO₂ NanoparticlesCe_((1-x-y))Zr_(x)Fe_(y)O_(2-δ).(x=0.15, 0.20; y=0.15-0.30)

The conditions of Example 7a were repeated; however the amount of ironor zirconium was adjusted to give the nominal stoichiometries indicatedin Table 3 below, using the appropriate metal containing salt solution,while the overall cerium nitrate hexahydrate was reduced to accommodatethe increased concentration of the iron or zirconium transition metalsalts in the reaction mixture.

Example 8 Precipitation of Iron Oxides (comparative)

The conditions of Example 4 were repeated, except that the metal saltssolution containing Ce(NO₃)₃.6H₂O was replaced with an equimolar amount(111.6 grams) of Fe(NO₃)₃.9H₂O. The product of the reaction was a turbidbrown solution which separated into a lower sediment portion and anupper portion that settled failed to clarify upon extended standing.

FIG. 5A is a TEM image representative of the particles prepared inExample 8, wherein an approximately 0.8 micrometer diameter ironcontaining oxide particle is shown. FIG. 5B is an electron diffractionpattern of the micron sized iron containing oxide particles prepared inExample 8. The electron diffraction peaks are most consistent withhydrated iron oxide phases known as iron oxyhydroxide.

Table 3 shown in FIG. 6 contains the particle size, Oxygen StorageCapacity and rate results for the homogeneously doped (Method 1) ceriumand iron-containing nanoparticles of Examples 4-7.

Comparison of the results shown in Table 3 of FIG. 6 for Example 4b toExample 4a shows a modest increase in OSC for nanoceria prepared usingMethod 1, wherein hydrogen peroxide oxidant and a molar ratio of 2.35methoxyacetic acid stabilizer to total metals were used, relative to thecommercial SigmaAldrich® cerium(IV) oxide nanopowder comparison.Comparison of Examples 5 and 6 to Examples 4a-4-b shows an OSC increaseof about 2.5× when iron is substituted for 10% of the cerium, and a 1.7×increase when zirconium is substituted for 15% of the cerium. Comparisonof Example 7 to Examples 4b, 5 and 6 indicates that the OSC increase dueto iron and zirconium co-doping is essentially additive relative to theeffect of each dopant metal alone. Comparison between Examples 7b and 7eshows that at a fixed zirconium level, an increasing amount of ironresults in a further increase in OSC. Comparison of Example 7f toExample 7c shows substantially the same OSC when the iron content is thesame. In summary, dramatic increases in OSC are seen when up to 40% ofthe cerium is replaced by zirconium, iron, or a combination thereof, innanoparticles prepared using homogeneous doping Method 1, whereinhydrogen peroxide oxidant and a molar ratio of methoxyacetic acidstabilizer to total metals of 2.35 were employed. In addition, irondoping has a stronger impact than zirconium in increasing OSC, althoughat lower doping levels the effects of the dopant metals on OSC areadditive.

Examples 9-10

These Examples used a nanoparticle preparation process (Method 2)comprising the sequential steps: 1) providing an aqueous solution ofmethoxyacetic acid stabilizer 2) concurrent addition of a first portionof metal salts along with addition of hydroxide ion, 3) addition ofhydrogen peroxide oxidant, 4) addition of a second portion of metalsalts; wherein the molar ratio of methoxyacetic acid stabilizer to totalmetals was 2.55.

Example 9 Preparation of Cerium Dioxide Nanoparticles (Inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 7.53 grams of cerium (III) nitrate hexahydratedissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of thereaction. Ammonium hydroxide was added until a pH of 4.5 was achieved atwhich time the addition was stopped. A 25 ml aqueous solution containing2.4 grams of 50 wt. % hydrogen peroxide was then pumped into thereaction flask at a rate of 5 ml/minute via a syringe pump. Aftercompletion of the hydrogen peroxide addition, a solution containing 7.53grams of cerium (III) nitrate hexahydrate dissolved in 10 ml ofdistilled water (total solution volume 10-11 ml) was added at a rate of3 ml/minute. The reaction mixture was an opaque dark orange brownishcolor at about a pH of 5. The reaction mixture was then heated for anadditional 60 minutes at 85° C., during which time the pH dropped to3.9, and the mixture became a clear yellow orange color. The reactionmixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water.

Examples 10a-10d Preparation of Homogeneously Doped Cerium DioxideNanoparticles: Ce_((1-x-y))Zr_(x)Fe_(y)O_(2-δ) (inventive) Example 10aPreparation of Ce_(0.85)Zr_(0.15)O_(2-δ)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 6.40 grams of cerium (III) nitrate hexahydrate and0.60 grams of zirconyl nitrate hydrate dissolved in 10 grams distilledwater (total solution volume of 10-11 ml), was drawn into a syringe pumpand then subsequently pumped at a rate of 3 ml/minute into the MAAcontaining flask. Concurrent with the start of the metal salt solutionaddition, an aliquot of about 10 ml of concentrated (28-30%) ammoniumhydroxide was pumped into the reaction vessel at a rate of 1.5ml/minute. The actual amount of ammonium hydroxide to be delivered isdependent on the desired pH of the reaction. Ammonium hydroxide wasadded until a pH of 4.5 was achieved at which time the addition wasstopped. A 25 ml aqueous solution containing 2.4 grams of 50 wt. %hydrogen peroxide was then pumped into the reaction flask at a rate of 5ml/minute via a syringe pump. After completion of the hydrogen peroxideaddition, a solution containing 6.40 grams of cerium (III) nitratehexahydrate and 0.60 grams of zirconyl nitrate hydrate dissolved in 10ml of distilled water (total solution volume 10-11 ml) was added at arate of 3 ml/minute. The reaction mixture was an opaque dark orangebrownish color at about a pH of 5. The reaction mixture was then heatedfor an additional 60 minutes at 85° C. degrees, during which time the pHdropped to 3.9, and the mixture became a clear yellow orange color. Thereaction mixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water.

Examples 10b Preparation of Ce_(0.6)Fe_(0.4)O_(2-δ)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 4.52 grams of cerium (III) nitrate hexahydrate and2.80 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. After completion ofthe hydrogen peroxide addition, a solution containing 4.52 grams ofcerium (III) nitrate hexahydrate and 2.80 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minute. Thereaction mixture was an opaque dark orange brownish color at about a pHof 5. The reaction mixture was then heated for an additional 60 minutesat 85° C. degrees, during which time the pH dropped to 3.9, and themixture became a clear yellow orange color. The reaction mixture wascooled with stirring overnight and diafiltered to a conductivity ofunder 10 mS/cm to remove excess water and unreacted materials. Thediafiltration process typically required an addition of about 500 ml ofdistilled water. A visual assessment of a TEM micrograph of particles ofthe final dispersion indicated a particle diameter of 3-5 nm.

Examples 10c Preparation of Ce_(0.45) Zr_(0.15)Fe_(0.40)O_(2-δ)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 3.39 grams of cerium (III) nitrate hexahydrate, 0.60grams of zirconyl nitrate hydrate and 2.80 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 grams distilled water(total solution volume of 10-11 ml), was drawn into a syringe pump andthen subsequently pumped at a rate of 3 ml/minute into the MAAcontaining flask. Concurrent with the start of the metal salt solutionaddition, an aliquot of about 10 ml of concentrated (28-30%) ammoniumhydroxide was pumped into the reaction vessel at a rate of 1.5ml/minute. The actual amount of ammonium hydroxide to be delivered isdependent on the desired pH of the reaction. Ammonium hydroxide wasadded until a pH of 4.5 was achieved at which time the addition wasstopped. A 25 ml aqueous solution containing 2.4 grams of 50 wt. %hydrogen peroxide was then pumped into the reaction flask at a rate of 5ml/minute via a syringe pump. After completion of the hydrogen peroxideaddition, a solution containing 3.39 grams of cerium (III) nitratehexahydrate, 0.60 grams of zirconyl nitrate hydrate and 2.80 grams ofiron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml ofdistilled water (total solution volume 10-11 ml) was added at a rate of3 ml/minute. The reaction mixture was an opaque dark orange brownishcolor at about a pH of 5. The reaction mixture was then heated for anadditional 60 minutes at 85° C. degrees, during which time the pHdropped to 3.8, and the mixture became a clear yellow orange color. Thereaction mixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water.

Examples 10d Preparation of Ce_(0.30) Zr_(0.30)Fe_(0.40)O_(2-δ)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 2.26 grams of cerium (III) nitrate hexahydrate, 1.20grams of zirconyl nitrate hydrate and 2.80 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 grams distilled water(total solution volume of 10-11 ml), was drawn into a syringe pump andthen subsequently pumped at a rate of 3 ml/minute into the MAAcontaining flask. Concurrent with the start of the metal salt solutionaddition, an aliquot of about 10 ml of concentrated (28-30%) ammoniumhydroxide was pumped into the reaction vessel at a rate of 1.5ml/minute. The actual amount of ammonium hydroxide to be delivered isdependent on the desired pH of the reaction. Ammonium hydroxide wasadded until a pH of 4.5 was achieved at which time the addition wasstopped. A 25 ml aqueous solution containing 2.4 grams of 50 wt. %hydrogen peroxide was then pumped into the reaction flask at a rate of 5ml/minute via a syringe pump. After completion of the hydrogen peroxideaddition, a solution containing 2.26 grams of cerium (III) nitratehexahydrate, 1.20 grams of zirconyl nitrate hydrate and 2.80 grams ofiron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml ofdistilled water (total solution volume 10-11 ml) was added at a rate of3 ml/minute. The reaction mixture was an opaque dark orange brownishcolor at about a pH of 5. The reaction mixture was then heated for anadditional 60 minutes at 85° C. degrees, during which time the pHdropped to 3.6, and the mixture became a clear yellow orange color. Thereaction mixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water.

Samples of the materials prepared as described above in Examples 9 and10a-10d were evaluated for OSC and rate by the procedures describedabove. Results are contained in Table 4.

TABLE 4 Rate Composition OSC Constant Ex. (Method 2) (μmoleO₂/g) ×10³(min⁻¹) Comment  9 CeO₂ 265.8 1.3 Inventive 10aCe_(0.85)Zr_(0.15)O_(2-δ) 855.6 3.9 Inventive 10bCe_(0.60)Fe_(0.40)O_(2-δ) 4461.2 24.5 Inventive 10cCe_(0.45)Zr_(0.15)Fe_(0.40)O_(2-δ) 4361.8 7.2 Inventive 10dCe_(0.30)Zr_(0.30)Fe_(0.40)O_(2-δ) 4313.6 10.7 Inventive

Comparison of the results shown above for Example 9 to those of Examples10a-10d shows a dramatic increase in both OSC and rate when 15-70% ofthe cerium is replaced by zirconium, iron, or a combination thereof, innanoparticles prepared using homogeneous doping Method 2, hydrogenperoxide oxidant, and a molar ratio of methoxyacetic acid stabilizer tototal metals of 2.55.

Examples 11-12

These Examples used a nanoparticle preparation process (Method 3)comprising the steps: 1) providing an aqueous solution of methoxyaceticacid stabilizer, 2) concurrent addition of a first portion of metalsalts along with addition of hydroxide ion, 3) addition of a firstportion hydrogen peroxide oxidant, 4) concurrent addition of a secondportion of metal salts along with a second portion of hydrogen peroxideoxidant; wherein the molar ratio of methoxyacetic acid stabilizer tototal metals was 2.55.

Example 11 Preparation of Cerium Dioxide Nanoparticles (Inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 7.53 grams of cerium (III) nitrate hexahydratedissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of thereaction. Ammonium hydroxide was added until a pH of 4.5 was achieved atwhich time the addition was stopped. A 25 ml aqueous solution containing2.4 grams of 50 wt. % hydrogen peroxide was then pumped into thereaction flask at a rate of 5 ml/minute via a syringe pump. At themidpoint of the hydrogen peroxide addition, a solution containing 7.53grams of cerium (III) nitrate hexahydrate dissolved in 10 ml ofdistilled water (total solution volume 10-11 ml) was added at a rate of3 ml/minute concurrently with the remaining half of the hydrogenperoxide. The reaction mixture was an opaque dark orange brownish colorat about a pH of 5. The reaction mixture was then heated for anadditional 60 minutes at 85° C. degrees, during which time the pHdropped to 3.9, and the mixture became a clear yellow orange color. Thereaction mixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water.

Examples 12a-12d Preparation of Homogeneously Doped Cerium DioxideNanoparticles Ce_((1-x-y))Zr_(x)Fe_(y)O_(2-δ) Method 3 (inventive)[MW-73] Example 12a Preparation of Ce_(0.85)Zr_(0.15)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 6.40 grams of cerium (III) nitrate hexahydrate and0.60 grams of zirconyl nitrate hydrate dissolved in 10 grams distilledwater (total solution volume of 10-11 ml), was drawn into a syringe pumpand then subsequently pumped at a rate of 3 ml/minute into the MAAcontaining flask. Concurrent with the start of the metal salt solutionaddition, an aliquot of about 10 ml of concentrated (28-30%) ammoniumhydroxide was pumped into the reaction vessel at a rate of 1.5ml/minute. The actual amount of ammonium hydroxide to be delivered isdependent on the desired pH of the reaction. Ammonium hydroxide wasadded until a pH of 4.5 was achieved at which time the addition wasstopped. A 25 ml aqueous solution containing 2.4 grams of 50 wt. %hydrogen peroxide was then pumped into the reaction flask at a rate of 5ml/minute via a syringe pump. At the midpoint of the hydrogen peroxideaddition, a solution containing 6.40 grams of cerium (III) nitratehexahydrate and 0.60 grams of zirconyl nitrate hydrate dissolved in 10ml of distilled water (total solution volume 10-11 ml) was added at arate of 3 ml/minute concurrently with the remaining half of the hydrogenperoxide. When all reagents had been added, the reaction mixture was anopaque dark orange brownish color at about a pH of 5. The reactionmixture was then heated for an additional 60 minutes at 85° C. degrees,during which time the pH dropped to 3.9, and the mixture became a clearyellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.

Example 12b Preparation of Ce_(0.60)Fe_(0.40)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 4.52 grams of cerium (III) nitrate hexahydrate and2.80 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. At the midpoint ofthe hydrogen peroxide addition, a solution containing 4.52 grams ofcerium (III) nitrate hexahydrate and 2.80 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minuteconcurrently with the remaining half of the hydrogen peroxide. When allreagents had been added, the reaction mixture was an opaque dark orangebrownish color at about a pH of 5. The reaction mixture was then heatedfor an additional 60 minutes at 85° C. degrees, during which time the pHdropped to 3.9, and the mixture became a clear yellow orange color. Thereaction mixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water. A visual assessment of a TEM micrographof particles of the final dispersion indicated a particle diameter of3-5 nm.

Example 12c Preparation of Ce_(0.45)Zr_(0.15)Fe_(0.40)O_(2-δ)(inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 3.39 grams of cerium (III) nitrate hexahydrate, 0.60grams of zirconyl nitrate hydrate and 2.80 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 grams distilled water(total solution volume of 10-11 ml), was drawn into a syringe pump andthen subsequently pumped at a rate of 3 ml/minute into the MAAcontaining flask. Concurrent with the start of the metal salt solutionaddition, an aliquot of about 10 ml of concentrated (28-30%) ammoniumhydroxide was pumped into the reaction vessel at a rate of 1.5ml/minute. The actual amount of ammonium hydroxide to be delivered isdependent on the desired pH of the reaction. Ammonium hydroxide wasadded until a pH of 4.5 was achieved at which time the addition wasstopped. A 25 ml aqueous solution containing 2.4 grams of 50 wt. %hydrogen peroxide was then pumped into the reaction flask at a rate of 5ml/minute via a syringe pump. At the midpoint of the hydrogen peroxideaddition, a solution containing 3.39 grams of cerium (III) nitratehexahydrate, 0.60 grams of zirconyl nitrate hydrate and 2.80 grams ofiron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml ofdistilled water (total solution volume 10-11 ml) was added at a rate of3 ml/minute concurrently with the remaining half of the hydrogenperoxide. When all reagents had been added, the reaction mixture was anopaque dark orange brownish color at about a pH of 5. The reactionmixture was then heated for an additional 60 minutes at 85° C. degrees,during which time the pH dropped to 3.9, and the mixture became a clearyellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.

Example 12d Preparation of Ce_(0.30)Zr_(0.30)Fe_(0.40)O_(2-δ)(inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 85° C. with constant bar stirring. A metal saltsolution containing 2.26 grams of cerium (III) nitrate hexahydrate, 1.20grams of zirconyl nitrate hydrate and 2.80 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 grams distilled water(total solution volume of 10-11 ml), was drawn into a syringe pump andthen subsequently pumped at a rate of 3 ml/minute into the MAAcontaining flask. Concurrent with the start of the metal salt solutionaddition, an aliquot of about 10 ml of concentrated (28-30%) ammoniumhydroxide was pumped into the reaction vessel at a rate of 1.5ml/minute. The actual amount of ammonium hydroxide to be delivered isdependent on the desired pH of the reaction. Ammonium hydroxide wasadded until a pH of 4.5 was achieved at which time the addition wasstopped. A 25 ml aqueous solution containing 2.4 grams of 50 wt. %hydrogen peroxide was then pumped into the reaction flask at a rate of 5ml/minute via a syringe pump. At the midpoint of the hydrogen peroxideaddition, a solution containing 2.26 grams of cerium (III) nitratehexahydrate, 1.20 grams of zirconyl nitrate hydrate and 2.80 grams ofiron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml ofdistilled water (total solution volume 10-11 ml) was added at a rate of3 ml/minute concurrently with the remaining half of the hydrogenperoxide. When all reagents had been added, the reaction mixture was anopaque dark orange brownish color at about a pH of 5. The reactionmixture was then heated for an additional 60 minutes at 85° C. degrees,during which time the pH dropped to 3.9, and the mixture became a clearyellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.

Samples of the materials prepared as described above in Examples 11 and12a-12d were evaluated for OSC and rate by the procedures describedabove. Results are contained in Table 5.

TABLE 5 Rate Composition OSC Constant Ex. (Method 3) (μmoleO₂/g) ×10³(min⁻¹) Comment 11 CeO₂ 270.2 1.9 Inventive 12aCe_(0.85)Zr_(0.15)O_(2-δ) 1052.4 5.5 Inventive 12bCe_(0.60)Fe_(0.40)O_(2-δ) 5044.1 12.9 Inventive 12cCe_(0.45)Zr_(0.15)Fe_(0.40)O_(2-δ) 4237.9 7.8 Inventive 12dCe_(0.30)Zr_(0.30)Fe_(0.40)O_(2-δ) 3759.8 6.0 Inventive

Comparison of the results for comparative Example 11 to those ofExamples 12a-12d shows a dramatic increase in both OSC and rate when15-70% of the cerium is replaced by zirconium, iron, or a combinationthereof, in nanoparticles prepared using homogeneous doping Method 3,hydrogen peroxide oxidant, and a molar ratio of methoxyacetic acidstabilizer to total metals of 2.55.

Examples 13-14 These Examples used a nanoparticle preparation process(Method 3) comprising the steps: 1) providing an aqueous solution ofmethoxyacetic acid stabilizer, 2) concurrent addition of a first portionof metal salts along with addition of hydroxide ion, 3) addition of afirst portion hydrogen peroxide oxidant, 4) concurrent addition of asecond portion of metal salts along with a second portion of hydrogenperoxide oxidant; wherein the molar ratio of methoxyacetic acidstabilizer to total metals was about 2.55. Example 13 Preparation ofCerium Dioxide Nanoparticles (inventive) Method 3 [MW-73]

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 7.53 grams of cerium (III) nitrate hexahydratedissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of thereaction. Ammonium hydroxide was added until a pH of 4.5 was achieved atwhich time the addition was stopped. A 25 ml aqueous solution containing2.4 grams of 50 wt. % hydrogen peroxide was then pumped into thereaction flask at a rate of 5 ml/minute via a syringe pump. At themidpoint of the hydrogen peroxide addition, a solution containing 7.53grams of cerium (III) nitrate hexahydrate dissolved in 10 ml ofdistilled water (total solution volume 10-11 ml) was added at a rate of3 ml/minute concurrently with the remaining half of the hydrogenperoxide. When all reagents had been added, the reaction mixture was anopaque dark orange brownish color at about a pH of 5. The reactionmixture was then heated for an additional 60 minutes at 65° C. degrees,during which time the pH dropped to 3.9, and the mixture became a clearyellow orange color. The reaction mixture was cooled with stirringovernight and diafiltered to a conductivity of under 10 mS/cm to removeexcess water and unreacted materials. The diafiltration processtypically required an addition of about 500 ml of distilled water.

Example 14a-14e Preparation of Homogeneously Doped Cerium DioxideNanoparticles Ce_(1-x)Fe_(x)O_(2-δ) (inventive) Example 14a Preparationof Ce_(0.6)Fe_(0.4)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 4.52 grams of cerium (III) nitrate hexahydrate and2.80 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. At the midpoint ofthe hydrogen peroxide addition, a solution containing 4.52 grams ofcerium (III) nitrate hexahydrate and 2.80 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minuteconcurrently with the remaining half of the hydrogen peroxide. When allreagents had been added, the reaction mixture was an opaque dark orangebrownish color at about a pH of 5. The reaction mixture was then heatedfor an additional 60 minutes at 65° C., during which time the pH droppedto 3.9, and the mixture became a clear yellow orange color. The reactionmixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water. Particle size analysis of the clearyellow orange dispersion by dynamic light scattering indicated ahydrodynamic diameter of about 12 nm.

Example 14b Preparation of Ce_(0.4)Fe_(0.6)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 3.01 grams of cerium (III) nitrate hexahydrate and4.20 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. At the midpoint ofthe hydrogen peroxide addition, a solution containing 3.01 grams ofcerium (III) nitrate hexahydrate and 4.20 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minuteconcurrently with the remaining half of the hydrogen peroxide. When allreagents had been added, the reaction mixture was an opaque dark orangebrownish color at about a pH of 5. The reaction mixture was then heatedfor an additional 60 minutes at 65° C., during which time the pH droppedto 3.9, and the mixture became a clear yellow orange color. The reactionmixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water.

Example 14c Preparation of Ce_(0.3)Fe_(0.7)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 2.26 grams of cerium (III) nitrate hexahydrate and4.90 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. At the midpoint ofthe hydrogen peroxide addition, a solution containing 2.26 grams ofcerium (III) nitrate hexahydrate and 4.90 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minuteconcurrently with the remaining half of the hydrogen peroxide. When allreagents had been added, the reaction mixture was an opaque dark orangebrownish color at about a pH of 5. The reaction mixture was then heatedfor an additional 60 minutes at 65° C., during which time the pH droppedto 3.9, and the mixture became a clear yellow orange color. The reactionmixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water. Particle size-frequency analysis bytransmission electron micrography revealed a mean particle size of3.7±1.0 nm, with size frequency distribution having a coefficient ofvariation, COV, (one standard deviation divided by the mean diameter) of27%. Alternatively, after several months of storage at room temperature,a particle size analysis of the clear yellow orange dispersion bydynamic light scattering indicated a hydrodynamic diameter of about 6nm.

Example 14d Preparation of Ce_(0.2)Fe_(0.8)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 1.505 grams of cerium (III) nitrate hexahydrate and5.60 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. At the midpoint ofthe hydrogen peroxide addition, a solution containing 1.505 grams ofcerium (III) nitrate hexahydrate and 5.60 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minuteconcurrently with the remaining half of the hydrogen peroxide. When allreagents had been added, the reaction mixture was an opaque dark orangebrownish color at about a pH of 5. The reaction mixture was then heatedfor an additional 60 minutes at 65° C., during which time the pH droppedto 3.9, and the mixture became a clear yellow orange color. The reactionmixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water. Particle size analysis of the clearyellow orange dispersion by dynamic light scattering indicated ahydrodynamic diameter of about 12 nm.

Example 14e Preparation of Ce_(0.1)Fe_(0.9)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 0.7525 grams of cerium (III) nitrate hexahydrate and6.30 grams of iron (III) nitrate nonahydrate, 98Fe(NO₃)₃.9H₂O, dissolvedin 10 grams distilled water (total solution volume of 10-11 ml), wasdrawn into a syringe pump and then subsequently pumped at a rate of 3ml/minute into the MAA containing flask. Concurrent with the start ofthe metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. At the midpoint ofthe hydrogen peroxide addition, a solution containing 0.7525 grams ofcerium (III) nitrate hexahydrate and 6.30 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minuteconcurrently with the remaining half of the hydrogen peroxide. When allreagents had been added, the reaction mixture was an opaque dark orangebrownish color at about a pH of 5. The reaction mixture was then heatedfor an additional 60 minutes at 65° C., during which time the pH droppedto 3.9, and the mixture became a clear yellow orange color. The reactionmixture was cooled with stirring overnight and diafiltered to aconductivity of under 10 mS/cm to remove excess water and unreactedmaterials. The diafiltration process typically required an addition ofabout 500 ml of distilled water. Particle size-frequency analysis bytransmission electron micrography revealed a mean particle size of2.8±0.9 nm, with size frequency distribution having a coefficient ofvariation, COV, (one standard deviation divided by the mean diameter) of32%. Alternatively, after several months of storage at room temperature,a particle size analysis of the clear yellow orange dispersion bydynamic light scattering indicated a hydrodynamic diameter of about 15nm.

Example 14f Preparation of Ce_(0.045)Fe_(0.955)O_(2-δ) (inventive)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 0.376 grams of cerium (III) nitrate hexahydrate and7.34 grams of iron (III) nitrate nonahydrate, 98% Fe(NO₃)₃.9H₂O,dissolved in 10 grams distilled water (total solution volume of 10-11ml), was drawn into a syringe pump and then subsequently pumped at arate of 3 ml/minute into the MAA containing flask. Concurrent with thestart of the metal salt solution addition, an aliquot of about 10 ml ofconcentrated (28-30%) ammonium hydroxide was pumped into the reactionvessel at a rate of 1.5 ml/minute. The actual amount of ammoniumhydroxide to be delivered is dependent on the desired pH of the reactionAmmonium hydroxide was added until a pH of 4.5 was achieved at whichtime the addition was stopped. A 25 ml aqueous solution containing 2.4grams of 50 wt. % hydrogen peroxide was then pumped into the reactionflask at a rate of 5 ml/minute via a syringe pump. At the midpoint ofthe hydrogen peroxide addition, a solution containing 0.376 grams ofcerium (III) nitrate hexahydrate and 7.34 grams of iron (III) nitratenonahydrate, 98% Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water(total solution volume 10-11 ml) was added at a rate of 3 ml/minuteconcurrently with the remaining half of the hydrogen peroxide. The molarratio of methoxyacetic acid stabilizer to total metals was 2.32. Whenall reagents had been added, the reaction mixture was an opaque darkorange brownish color at about a pH of 5. The reaction mixture was thenheated for an additional 60 minutes at 65° C. degrees, during which timethe pH dropped to 3.9, and the mixture became a slightly turbid orangebrown color. The reaction mixture was cooled with stirring overnight.After standing unstirred for several hours, a small amount of a tancolored sediment appeared, above which the solution remained a cleardeep brown color. Particle size analysis of the clear deep brownsupernatant by dynamic light scattering indicated a hydrodynamicdiameter of about 21 nm.

Example 15 Preparation of Iron Oxides (Comparative)

To a 600 ml Erlenmeyer flask containing a one inch magnetic stir bar,8.13 grams of (98%) methoxyacetic acid (MAA) and 130 ml of distilledwater were introduced. The flask was then placed into a water bath at atemperature of about 65° C. with constant bar stirring. A metal saltsolution containing 7.35 grams of iron (III) nitrate nonahydrate, 98%Fe(NO₃)₃.9H₂O, dissolved in 10 grams distilled water (total solutionvolume of 10-11 ml), was drawn into a syringe pump and then subsequentlypumped at a rate of 3 ml/minute into the MAA containing flask.Concurrent with the start of the iron salt solution addition, an aliquotof about 10 ml of concentrated (28-30%) ammonium hydroxide was pumpedinto the reaction vessel at a rate of 1.5 ml/minute. The actual amountof ammonium hydroxide to be delivered is dependent on the desired pH ofthe reaction. Ammonium hydroxide was added until a pH of 4.5 wasachieved at which time the addition was stopped. A 25 ml aqueoussolution containing 2.4 grams of 50 wt. % hydrogen peroxide was thenpumped into the reaction flask at a rate of 5 ml/minute via a syringepump. At the midpoint of the hydrogen peroxide addition, a solutioncontaining 7.35 grams of iron (III) nitrate nonahydrate, 98%Fe(NO₃)₃.9H₂O, dissolved in 10 ml of distilled water (total solutionvolume 10-11 ml) was added at a rate of 3 ml/minute concurrently withthe remaining half of the hydrogen peroxide. When all reagents had beenadded, the reaction mixture was a turbid orange brown color. Thereaction mixture was then heated for an additional 60 minutes at 65° C.degrees. The reaction mixture was cooled, and after standing unstirredfor several hours, a light brown sediment occupied the bottom third ofthe reaction vessel, while the top portion was a slightly turbid deepbrown color. The molar ratio of methoxyacetic acid stabilizer to totalmetals was 2.43.

Particle size analysis of the slightly turbid deep brown supernatant bydynamic light scattering indicated a hydrodynamic diameter of about 28nm. Particle size analysis of a dispersion of the light brown sedimentby dynamic light scattering indicated a hydrodynamic diameter of about176 nm. These results are substantially similar to those obtained abovein Example 8, wherein iron oxides were prepared using the Method 1procedures of Example 4.

Samples of the materials prepared as described above in Examples 13 and14a-14e were evaluated for OSC and rate by the procedures describedabove. Results are contained in Table 6 below.

TABLE 6 Rate Composition OSC Constant Ex. (Method 3) (μmoleO₂/g) ×10³(min⁻¹) Comment 13 CeO₂ 215.4 6.0 Inventive 14a Ce_(0.6)Fe_(0.4)O_(2-δ)4651.7 21.5 Inventive 14b Ce_(0.4)Fe_(0.6)O_(2-δ) 5588.3 21.2 Inventive14c Ce_(0.3)Fe_(0.7)O_(2-δ) 6436.8 11.0 Inventive 14dCe_(0.2)Fe_(0.8)O_(2-δ) 7528.1 21.2 Inventive 14eCe_(0.1)Fe_(0.9)O_(2-δ) 7999.0 20.4 Inventive

Comparison of the results for Example 13 to those of Examples 14a-14eshows a dramatic increase in both OSC and rate when 40-95.5% of thecerium is replaced by iron in nanoparticles prepared using homogeneousdoping Method 3, hydrogen peroxide oxidant, and a molar ratio ofmethoxyacetic acid stabilizer to total metals of about 2.55.

Example 16 Ce_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ): Approaching the minimumcerium concentration at which a ceria cubic XRD lattice pattern wasobserved. [CeO-324b]

Example 16 was prepared according to the homogeneous doping proceduresof Method 1 as described in Example 4, except that an appropriate amountof zirconyl nitrate hydrate was added along with the cerium and ironsalts such that a nominal composition ofCe_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ) was produced. FIG. 7A shows a highresolution TEM image of the nanoparticles of homogeneously preparedCe_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ). FIG. 7B shows an electrondiffraction pattern of Ce_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ), in which areseen the characteristic Bragg reflections of the cubic fluorite CeO₂structure, thereby demonstrating the crystalline nature of thehomogeneously prepared Ce_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ)nanoparticulate material.

FIG. 7C shows the powder x-ray diffraction pattern of homogeneouslyprepared Ce_(0.35)Zr_(0.15)Fe_(0.50)O_(2-δ) superimposed on pure CeO₂(line structure). A small contraction of the ceria lattice spacing,consistent with incorporation of either of the smaller ionic radii Zr⁴⁺or Fe³⁺ ions, is indicated by the shift to higher 20 for a majority ofthe peak positions.

Example 17 Preparation of Homogeneously Doped Cerium DioxideCe_(0.6)Fe_(0.40)O_(2-δ)

To a 11 liter round bottom Type-316 stainless steel kettle or reactorwith 3 mixing baffles, was added 1144 grams of distilled water (KettleWater), which was maintained at 70° C. Using an impeller, the water wasstirred at sufficient speed to provide good mixing. Then 292.1 grams of98% methoxyacetic acid was added to the reactor. Two solutionintroduction jets directed to the impeller blades were put into thereactor and secured. An ammonium hydroxide solution (346.6 ml of 28-30%NH₄OH) was pumped through one jet at a rate of 69.3 ml/minute. Acerium-iron containing solution (334.5 gram of Ce(NO₃)₃.6H₂O and 207.5gram of Fe(NO₃)₃.9H₂O with distilled water to make 625 ml) was pumpedthrough the other jet at a delivery rate of 125 ml/minute. Thecerium-iron solution and the ammonium hydroxide solution each requiredabout 5 minutes to be delivered to the reactor. The ammonium hydroxidepump was stopped when the pH in the reactor reached about 4.5. Thecerium-iron solution was purged from the delivery line with a 15 mldistilled water chase. Then 93.8 gram of a 50% H₂O₂ solution was pumpedinto the reactor at 9.38 ml/minute using a third jet and was followed bya brief distilled water flush. For the first one-half of the peroxideaddition the pH was maintained at 4.5 and then allowed to drift downwardfreely for the remainder of the peroxide addition. The reaction mixturewas held at 70° C. for an additional 60 minutes after which time it wascooled to 20 C. The reaction was filtered and concentrated viadiafiltration (5000 Dalton filter) using 18 megaohm deionized water to afinal concentration of 5 mS/cm.

The reaction as described above employed a molar ratio of methoxyaceticacid stabilizer to total metals of 2.48. TEM grain sizing revealed aparticle size of 2.5±0.5 nm that had a distinct CeO₂ cubic fluoriteelectron diffraction pattern. This material was nanocrystalline and hada OSC of 4601.2 micromoles O₂/g and a rate constant of 15.6×10⁻³ min⁻¹.

Example 18 Preparation of Fuel Additive Concentrates

The procedures of Example 17 were repeated at a 16× larger scale withsimilar results. After diafiltration and concentration, the dispersionof stabilized cerium and iron containing nanoparticles was solventshifted by diafiltration procedures previously described in general inU.S. patent application Ser. No. 12/549,776, PROCESS FOR SOLVENTSHIFTING A NANOPARTICLE DISPERSION; such that a stable dispersioncontaining less than about 5% water and about 8% by weight of thestabilized nanoparticles in a 1:1 by volume mixture of diethylene glycolmonomethyl ether and 1-methoxy-2-propanol was produced. A portion of thesolvent shifted dispersion was combined with a 1:2 by volume mixture ofoleic acid surfactant and kerosene diluent, such that a stabledispersion containing about 2% by weight of the stabilized nanoparticleswas produced.

Example 19 Preparation of Additivized Diesel Fuel Containing FuelAdditive

Additivized diesel fuel containing about 5 parts per million (ppm) ofthe stabilized cerium and iron containing nanoparticles was prepared byadding 1 part of the fuel additive concentrate prepared in Example 18 toabout 4000 parts of diesel fuel.

Example 20 Pushboat Evaluation of Additivized Diesel Fuel

The additivized diesel fuel prepared as described in Example 19 wasevaluated in a Cummins KTA1150 inline 6-cylinder 4-cycle turbochargedengine with a displacement of 19 L and rated power of 500 HP, using testmethodology adapted, in part, from SAE J1321 (1986-10) JOINT TMC/SAEFUEL COMSUMPTION TEST PROCEDURE—TYPE II, which is directed specificallytoward comparison of fuels by testing in truck engines. The test enginewas one of two identical propulsion engines onboard a commercialpushboat that is used to push barges filled with commodities such asgrain, stone, coal, etc., along commercial shipping waterways. Eachengine was supplied with fuel from its own dedicated fuel tank. Duringthe Baseline Segment, both engines were supplied with untreated fuel.During the Test Segment, the supply of fuel to one engine (Treat Engine(T)) was treated with the additive, while the supply of fuel to theother engine (Control Engine (C)) remained untreated diesel fuel. Thismethodology allowed for one engine to remain untreated and be used as acontrol to detect and compensate for any environmental factors whichcould not be controlled throughout the evaluation.

Each engine was instrumented with its own Sensors Inc. (Saline, Mich.,USA) SEMTECH-DS Portable Emissions Measurement System (PEMS) thatmeasured exhaust emissions of CO₂, CO, NO₂, NO, and THC; from which fuelconsumption was calculated using a carbon balance analysis method. EachPEMS continuously measured exhaust emissions and fuel consumption at 1Hz sampling. The operation of the Control Engine and its PEMS analyzerwas completely independent from the Treat Engine operation and its PEMSanalyzer.

Each Test Run consisted of running both engines simultaneously at afixed RPM either with the pushboat stationary against a dock or on afixed route on an inland waterway which had no tide or current. Prior toeach Test Run, the engines were warmed up for at least two hours, andthen the Test Run began with the PEMS recording gas emissions and fuelmeasurements of both engines simultaneously under the steady state 1400RPM load for approximately 20 minutes.

Test Runs were conducted on the engines before any additized fuel wasused to establish baseline performance of both the Control and Treatengines; these runs comprised the Baseline Segment. Test Runs were thenrepeated after the Treat engine's fuel had been additized such that theconcentration of the active doped cerium dioxide species (e.g. ceriumand iron containing nanoparticles) was 5 ppm by weight in the dieselfuel. The Control engine's fuel remained untreated. Test Runs duringwhich the Treat engine's fuel was additized comprised the Test Segment.

For each Test Run, T/C Ratios were calculated for fuel consumption andeach gas species emission. A T/C Ratio is the ratio of the measuredvalue of a PEMS parameter—such as fuel consumption—for the Treat Engine(T) to the measured value of the same parameter for the Control Engine(C) for that Test Run. T/C Ratios establish the relative performance ofthe Treat Engine to the Control Engine and incorporate the ControlEngine's role of compensating for environmental factors which affectedboth engines. For each PEMS parameter, the T/C Ratio for each Test Runduring the Test Segment was compared to the average T/C Ratio for thatparameter during the Baseline Segment. The percent difference in a PEMSparameter's Test Segment T/C Ratio versus its average Baseline SegmentT/C Ratio represents the effect the diesel fuel additive had on theTreat Engine's performance for that PEMS parameter. An improvement(reduction) in a PEMS parameter such as fuel consumption or a gasspecies emission is represented as a negative value. Table 7 belowcontains the pushboat fuel consumption results for various Test Runsduring the Treat Segment (after additivization), expressed as a percentchange from the Baseline Segment level.

TABLE 7 Hours After Percent Change in Percent Change in Treatment FuelConsumption NO Gas Emission 86 −2.0 0.1 94 −8.8 −14.1 120 −9.2 −13.7 136−8.1 −16.0

The results shown in Table 7 indicate that after 86 hours of engineoperation during the Treat Segment, the Treat Engine began to show asignificant improvement (reduction) in fuel consumption. Thereafter,during the Test Runs following 94-136 hours of engine operation, theTreat Engine displayed an 8-9% improvement (reduction) in fuelconsumption and a 14-16% reduction in nitric oxide (NO) gas emission asa result of the addition of 5 ppm of the cerium and iron containingnanoparticles in the diesel fuel.

Example 21 Preparation of Homogeneously Doped Cerium DioxideCe_(0.45)Zr_(0.15)Fe_(0.40)O_(2-δ)

The procedures of Example 17 were repeated, except that the temperatureof the reaction was increased to 85° C., and the amounts of metal ionsalts employed was as follows: 250.9 grams of Ce(NO₃)₃.6H₂O, 65.35 gramsof zirconyl nitrate hydrate 207.5 and 207.5 grams of Fe(NO₃)₃.9H₂O. Thusa molar metal ions composition of 45% cerium, 15% zirconium and 40% ironwas used.

The reaction as described above employed an oxidant, 94.9 gram of a 50%H₂O₂ solution, and a molar ratio of methoxyacetic acid stabilizer tototal metals of 2.31. This material had a OSC of 4723 micromoles O₂/gand a rate constant of 9.6×10⁻³ min⁻¹.

Example 22 Preparation of Fuel Additive Concentrate

After diafiltration and concentration, the dispersion of stabilizedcerium, zirconium and iron containing nanoparticles prepared asdescribed in Example 21 was solvent shifted by diafiltration proceduresdescribed in Example 18, such that a stable dispersion of the stabilizednanoparticles in a 1:1 by volume mixture of diethylene glycol monomethylether and 1-methoxy-2-propanol, which contained less than about 5%water, was produced. A portion of the solvent shifted dispersion wascombined with oleic acid surfactant, and then combined with kerosenediluent, such that the volume ratio of oleic acid to kerosene used was1:2. The resulting fuel additive concentrate was a stable dispersioncontaining about 2% by weight of the stabilized cerium, zirconium andiron containing nanoparticles.

Example 23 Preparation of Additivized Diesel Fuel

Additivized diesel fuel containing about 5 parts per million (ppm) ofthe stabilized cerium, zirconium and iron containing nanoparticles wasprepared by adding 1 part of the fuel additive concentrate prepared inExample 22 to about 4000 parts of diesel fuel.

Example 24 Pushboat Evaluation of Additivized Diesel Fuel

The additivized diesel fuel prepared as described in Example 23 wasevaluated in a Detroit Diesel 671 inline 6-cylinder 2-cycle Roots blowerfed engine with a displacement of 7 L and rated power of 238 HP, usingtest methodology adapted, in part, from SAE J1321 (1986-10) JOINTTMC/SAE FUEL COMSUMPTION TEST PROCEDURE—TYPE II, as described above inExample 20. Once more, the test engine was one of two identicalpropulsion engines onboard a commercial pushboat that is used to pushbarges filled with commodities such as grain, stone, coal, etc., alongcommercial shipping waterways. Each engine was supplied with fuel fromits own dedicated fuel tank.

Table 8 contains the pushboat fuel consumption results for various TestRuns during the Treat Segment (after additivization), expressed as apercent change from the Baseline Segment level.

TABLE 8 Hours After Percent Change in Percent Change in Treatment FuelConsumption NO Gas Emission 3 −3.0 −4.0 24 −5.0 −7.0 48 −8.0 −11.0 72−7.0 −11.0 96 −7.0 −5.0

The results indicate that after 3 hours and 24 hours of engine operationduring the Treat Segment, the Treat Engine began to show improvements(reductions) in fuel consumption of 3.0% and 5.0%, respectively.Thereafter, during the Test Runs following 48-96 hours of engineoperation, the Treat Engine displayed a 7-8% improvement (reduction) infuel consumption. Similar improvements were also observed for reductionin nitric oxide (NO) gas emission. After 3 hours and 24 hours of engineoperation during the Treat Segment, the Treat Engine showed 4% and a7.0% reduction in NO gas emission, respectively. Following 48-72 hoursof engine operation, the Treat Engine displayed an 11% improvement(reduction) in NO gas emission as a result of the addition of 5 ppm ofthe cerium, zirconium and iron containing nanoparticles in the dieselfuel.

While the invention has been described by reference to various specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

1. A process for making cerium oxide nanoparticles comprising: formingan aqueous reaction mixture comprising a source of cerous ion, a sourceof hydroxide ion, at least one monoether carboxylic acid nanoparticlestabilizer, wherein the molar ratio of said monoether carboxylic acidnanoparticle stabilizers to total metal ions is greater than 0.2, and anoxidant at an initial temperature in the range of about 20° C. to about100° C.; and providing temperature conditions effective to enableoxidation of the cerous ion to ceric ion, thereby forming a productdispersion comprising cerium oxide nanoparticles, CeO_(2-δ).
 2. Aprocess for making cerium-containing oxide nanoparticles containing atleast one other metal (M), said process comprising: forming an aqueousreaction mixture comprising a source of cerous ion and a source of oneor more ions of a metal (M) other than cerium, wherein said sources ofmetal ions are introduced concurrently, a source of hydroxide ion, atleast one monoether carboxylic acid nanoparticle stabilizer, wherein themolar ratio of said monoether carboxylic acid nanoparticle stabilizersto total metal ions is greater than 0.2, and an oxidant at an initialtemperature in the range of about 20° C. to about 100° C.; and providingtemperature conditions effective to enable oxidation of the cerous ionto ceric ion, thereby forming a product dispersion comprisingmetal-containing cerium oxide nanoparticles, Ce_(1-x)M_(x)O_(2-δ),wherein “x” has a value from about 0.01 to about 0.95.
 3. The processaccording to claim 1, wherein said molar ratio of said monoethercarboxylic acid nanoparticle stabilizers to total metal ions is greaterthan about 0.25.
 4. The process according to claim 1, wherein saidmonoether nanoparticle stabilizer is selected from the group consistingof ethoxyacetic acid, methoxyacetic acid, 3-methoxypropionic acid, andcombinations thereof.
 5. The process according to claim 4, wherein saidmonoether nanoparticle stabilizer is methoxyacetic acid.
 6. The processaccording to claim 1, wherein said oxidant comprises hydrogen peroxide.7. The process according to claim 2, wherein said other metal M in saidother metal-containing cerium oxide nanoparticles, Ce_(1-x)M_(x)O₂, isselected from the group consisting of transition metals, rare earthmetals, and combinations thereof.
 8. The process according to claim 7,wherein said other metal M in said other metal-containing cerium oxidenanoparticles, Ce_(1-x)M_(x)O₂, is selected from the group consisting ofiron, zirconium, and combinations thereof.
 9. The process according toclaim 1, wherein “x” has a value of about 0.10 to about 0.90.
 10. Theprocess according to claim 1, wherein “x” has a value of about 0.30 toabout 0.60.
 11. The process according to claim 1, wherein saidnanoparticles are characterized by a crystalline cubic fluoritestructure.
 12. The process according to claim 1, wherein saidnanoparticles are characterized by a mean hydrodynamic diameter lessthan about 50 nm.
 13. The process according to claim 12, wherein saidnanoparticles are characterized by a mean hydrodynamic diameter lessthan about 20 nm.
 14. The process according to claim 1 wherein saidnanoparticles are characterized by a mean geometric (TEM) diameter ofless than about 45 nm.
 15. The process according to claim 14, whereinsaid nanoparticles are characterized by a mean geometric (TEM) diameterof less than about 15 nm.
 16. The process according to claim 1, whereinsaid nanoparticles are characterized by a substantially monomodal sizedistribution and a substantially monodisperse size frequencydistribution.
 17. The process according to claim 1, wherein said sourceof cerous ion comprises cerous nitrate.
 18. The process according toclaim 1 wherein said source of hydroxide ion comprises ammoniumhydroxide, said hydroxide ion being in a molal stoichiometric ratiorelative to cerous ion of about 2:1 OH:Ce to about 5:1 OH:Ce.
 19. Theprocess according to claim 1 wherein said process is a batch process ora continuous process.
 20. The process according to claim 1, wherein saidtemperature conditions effective to enable oxidation of cerous ion toceric ion comprise a temperature less than or equal to 100° C.
 21. Theprocess according to claim 20, wherein said temperature conditionseffective to enable oxidation of cerous ion to ceric ion comprise atemperature from about 60° C. to about 90° C.
 22. The process accordingto claim 1, wherein the pH of said reaction mixture is less than orequal to
 7. 23. The process according to claim 1, wherein the pH of saidreaction mixture is less than or equal to about
 5. 24. The processaccording to claim 1, wherein the pH of said reaction mixture is lessthan or equal to about 4.5.
 25. The process according to claim 1 furthercomprising the sequential steps of 1) providing an aqueous solution ofthe at least one monoether carboxylic acid nanoparticle stabilizer, 2)concurrently adding said source of cerous ion and said source ofhydroxide ion to the aqueous solution of the at least one monoethercarboxylic acid nanoparticle stabilizer, 3) adding an oxidant to thereaction mixture provided by step
 2. 26. The process according to claim25, wherein the oxidant is hydrogen peroxide.
 27. The process accordingto claim 1, further comprising the steps of: 1) providing an aqueoussolution of said stabilizer; 2) concurrently adding a first portion ofsaid source of cerous ion and said source of hydroxide ion to thereaction mixture provided by step 1; 3) adding an oxidant to thereaction mixture provided by step 2; and 4) adding a second portion ofsaid source of cerous ion to the reaction mixture provided by step 3.28. The process according to claim 27, wherein said oxidant is hydrogenperoxide.
 29. The process according to claim 1, further comprising thesteps of: 1) providing an aqueous solution of said stabilizer; 2)concurrently adding a first portion of said source of cerous ion andsaid source of hydroxide ion to the reaction mixture provided by step 1;3) adding a first portion of an oxidant to the reaction mixture providedby step 2; and 4) concurrently adding a second portion of said source ofcerous ion and a second portion of said oxidant to the reaction mixtureprovided by step
 3. 30. The process according to claim 29, wherein saidoxidant is hydrogen peroxide.
 31. A fuel comprising cerium-containingoxide nanoparticles made according to the process of claim
 1. 32. A fuelcomprising metal-containing cerium oxide nanoparticles made according tothe process of claim
 2. 33. The fuel according to claim 32, wherein thefuel consumption of an engine powered by said fuel is reduced by greaterthan about 8 percent.
 34. A fuel comprising a dispersion ofcerium-containing oxide nanoparticles, wherein the fuel consumption ofan engine powered by said fuel is reduced by greater than about 8percent relative to said fuel not containing said dispersion.
 35. A fuelcomprising a dispersion of metal-containing cerium oxide nanoparticles,wherein the metal is other than rhodium or palladium, and wherein thefuel consumption of an engine powered by said fuel is reduced by greaterthan about 8 percent relative to said fuel not containing saiddispersion.
 36. A fuel comprising a dispersion of metal-containingcerium oxide nanoparticles, wherein the metal is other than rhodium orpalladium, and wherein the nitric oxide gas emission of an enginepowered by said fuel is reduced by greater than about 14 percentrelative to said fuel not containing said dispersion.